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/* Evaluator for GNU Emacs Lisp interpreter.
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   Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1999, 2000, 2001,
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		 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
		 Free Software Foundation, Inc.
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This file is part of GNU Emacs.

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GNU Emacs is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
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GNU Emacs is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
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along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
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#include <config.h>
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#include <setjmp.h>
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#include "lisp.h"
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#include "blockinput.h"
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#include "commands.h"
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#include "keyboard.h"
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#include "dispextern.h"
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#include "frame.h"		/* For XFRAME.  */
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#if HAVE_X_WINDOWS
#include "xterm.h"
#endif

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/* This definition is duplicated in alloc.c and keyboard.c */
/* Putting it in lisp.h makes cc bomb out! */

struct backtrace
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{
  struct backtrace *next;
  Lisp_Object *function;
  Lisp_Object *args;	/* Points to vector of args. */
  int nargs;		/* Length of vector.
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			   If nargs is UNEVALLED, args points to slot holding
			   list of unevalled args */
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  char evalargs;
  /* Nonzero means call value of debugger when done with this operation. */
  char debug_on_exit;
};
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struct backtrace *backtrace_list;

struct catchtag *catchlist;

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#ifdef DEBUG_GCPRO
/* Count levels of GCPRO to detect failure to UNGCPRO.  */
int gcpro_level;
#endif

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Lisp_Object Qautoload, Qmacro, Qexit, Qinteractive, Qcommandp, Qdefun;
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Lisp_Object Qinhibit_quit, Vinhibit_quit, Vquit_flag;
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Lisp_Object Qand_rest, Qand_optional;
Lisp_Object Qdebug_on_error;
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Lisp_Object Qdeclare;
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Lisp_Object Qcurry;
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Lisp_Object Qinternal_interpreter_environment, Qclosure;

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Lisp_Object Qdebug;
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/* This holds either the symbol `run-hooks' or nil.
   It is nil at an early stage of startup, and when Emacs
   is shutting down.  */
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Lisp_Object Vrun_hooks;

/* Non-nil means record all fset's and provide's, to be undone
   if the file being autoloaded is not fully loaded.
   They are recorded by being consed onto the front of Vautoload_queue:
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   (FUN . ODEF) for a defun, (0 . OFEATURES) for a provide.  */
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Lisp_Object Vautoload_queue;

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/* When lexical binding is being used, this is non-nil, and contains an
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   alist of lexically-bound variable, or (t), indicating an empty
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   environment.  The lisp name of this variable is
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   `internal-interpreter-environment'.  Every element of this list
   can be either a cons (VAR . VAL) specifying a lexical binding,
   or a single symbol VAR indicating that this variable should use
   dynamic scoping.  */
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Lisp_Object Vinternal_interpreter_environment;

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/* Current number of specbindings allocated in specpdl.  */
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EMACS_INT specpdl_size;
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/* Pointer to beginning of specpdl.  */
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struct specbinding *specpdl;

/* Pointer to first unused element in specpdl.  */
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struct specbinding *specpdl_ptr;
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/* Maximum size allowed for specpdl allocation */
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EMACS_INT max_specpdl_size;
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/* Depth in Lisp evaluations and function calls.  */
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EMACS_INT lisp_eval_depth;
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/* Maximum allowed depth in Lisp evaluations and function calls.  */
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EMACS_INT max_lisp_eval_depth;
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/* Nonzero means enter debugger before next function call */
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int debug_on_next_call;

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/* Non-zero means debugger may continue.  This is zero when the
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   debugger is called during redisplay, where it might not be safe to
   continue the interrupted redisplay. */

int debugger_may_continue;

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/* List of conditions (non-nil atom means all) which cause a backtrace
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   if an error is handled by the command loop's error handler.  */
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Lisp_Object Vstack_trace_on_error;
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/* List of conditions (non-nil atom means all) which enter the debugger
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   if an error is handled by the command loop's error handler.  */
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Lisp_Object Vdebug_on_error;
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/* List of conditions and regexps specifying error messages which
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   do not enter the debugger even if Vdebug_on_error says they should.  */
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Lisp_Object Vdebug_ignored_errors;

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/* Non-nil means call the debugger even if the error will be handled.  */
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Lisp_Object Vdebug_on_signal;
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/* Hook for edebug to use.  */
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Lisp_Object Vsignal_hook_function;

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/* Nonzero means enter debugger if a quit signal
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   is handled by the command loop's error handler. */
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int debug_on_quit;

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/* The value of num_nonmacro_input_events as of the last time we
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   started to enter the debugger.  If we decide to enter the debugger
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   again when this is still equal to num_nonmacro_input_events, then we
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   know that the debugger itself has an error, and we should just
   signal the error instead of entering an infinite loop of debugger
   invocations.  */
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int when_entered_debugger;
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Lisp_Object Vdebugger;

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/* The function from which the last `signal' was called.  Set in
   Fsignal.  */

Lisp_Object Vsignaling_function;

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/* Set to non-zero while processing X events.  Checked in Feval to
   make sure the Lisp interpreter isn't called from a signal handler,
   which is unsafe because the interpreter isn't reentrant.  */

int handling_signal;

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/* Function to process declarations in defmacro forms.  */

Lisp_Object Vmacro_declaration_function;

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static Lisp_Object apply_lambda (Lisp_Object fun, Lisp_Object args);
static Lisp_Object funcall_lambda (Lisp_Object, int, Lisp_Object *);
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static void unwind_to_catch (struct catchtag *, Lisp_Object) NO_RETURN;
184

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void
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init_eval_once (void)
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{
  specpdl_size = 50;
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  specpdl = (struct specbinding *) xmalloc (specpdl_size * sizeof (struct specbinding));
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  specpdl_ptr = specpdl;
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  /* Don't forget to update docs (lispref node "Local Variables").  */
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  max_specpdl_size = 1200; /* 1000 is not enough for CEDET's c-by.el.  */
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  max_lisp_eval_depth = 600;
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  Vrun_hooks = Qnil;
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}

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void
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init_eval (void)
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{
  specpdl_ptr = specpdl;
  catchlist = 0;
  handlerlist = 0;
  backtrace_list = 0;
  Vquit_flag = Qnil;
  debug_on_next_call = 0;
  lisp_eval_depth = 0;
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#ifdef DEBUG_GCPRO
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  gcpro_level = 0;
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#endif
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  /* This is less than the initial value of num_nonmacro_input_events.  */
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  when_entered_debugger = -1;
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}

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/* unwind-protect function used by call_debugger.  */

static Lisp_Object
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restore_stack_limits (Lisp_Object data)
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{
  max_specpdl_size = XINT (XCAR (data));
  max_lisp_eval_depth = XINT (XCDR (data));
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  return Qnil;
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}

/* Call the Lisp debugger, giving it argument ARG.  */

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Lisp_Object
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call_debugger (Lisp_Object arg)
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{
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  int debug_while_redisplaying;
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  int count = SPECPDL_INDEX ();
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  Lisp_Object val;
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  EMACS_INT old_max = max_specpdl_size;
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  /* Temporarily bump up the stack limits,
     so the debugger won't run out of stack.  */
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  max_specpdl_size += 1;
  record_unwind_protect (restore_stack_limits,
			 Fcons (make_number (old_max),
				make_number (max_lisp_eval_depth)));
  max_specpdl_size = old_max;

  if (lisp_eval_depth + 40 > max_lisp_eval_depth)
    max_lisp_eval_depth = lisp_eval_depth + 40;

  if (SPECPDL_INDEX () + 100 > max_specpdl_size)
    max_specpdl_size = SPECPDL_INDEX () + 100;
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#ifdef HAVE_WINDOW_SYSTEM
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  if (display_hourglass_p)
    cancel_hourglass ();
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#endif

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  debug_on_next_call = 0;
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  when_entered_debugger = num_nonmacro_input_events;
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  /* Resetting redisplaying_p to 0 makes sure that debug output is
     displayed if the debugger is invoked during redisplay.  */
  debug_while_redisplaying = redisplaying_p;
  redisplaying_p = 0;
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  specbind (intern ("debugger-may-continue"),
	    debug_while_redisplaying ? Qnil : Qt);
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  specbind (Qinhibit_redisplay, Qnil);
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  specbind (Qdebug_on_error, Qnil);
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#if 0 /* Binding this prevents execution of Lisp code during
	 redisplay, which necessarily leads to display problems.  */
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  specbind (Qinhibit_eval_during_redisplay, Qt);
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#endif
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  val = apply1 (Vdebugger, arg);

  /* Interrupting redisplay and resuming it later is not safe under
     all circumstances.  So, when the debugger returns, abort the
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     interrupted redisplay by going back to the top-level.  */
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  if (debug_while_redisplaying)
    Ftop_level ();

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  return unbind_to (count, val);
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}

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void
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do_debug_on_call (Lisp_Object code)
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{
  debug_on_next_call = 0;
  backtrace_list->debug_on_exit = 1;
  call_debugger (Fcons (code, Qnil));
}

/* NOTE!!! Every function that can call EVAL must protect its args
   and temporaries from garbage collection while it needs them.
   The definition of `For' shows what you have to do.  */

DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
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       doc: /* Eval args until one of them yields non-nil, then return that value.
The remaining args are not evalled at all.
If all args return nil, return nil.
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usage: (or CONDITIONS...)  */)
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  (Lisp_Object args)
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{
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  register Lisp_Object val = Qnil;
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  struct gcpro gcpro1;

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  GCPRO1 (args);
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  while (CONSP (args))
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    {
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      val = eval_sub (XCAR (args));
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      if (!NILP (val))
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	break;
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      args = XCDR (args);
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    }

  UNGCPRO;
  return val;
}

DEFUN ("and", Fand, Sand, 0, UNEVALLED, 0,
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       doc: /* Eval args until one of them yields nil, then return nil.
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The remaining args are not evalled at all.
If no arg yields nil, return the last arg's value.
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usage: (and CONDITIONS...)  */)
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  (Lisp_Object args)
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{
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  register Lisp_Object val = Qt;
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  struct gcpro gcpro1;

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  GCPRO1 (args);
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  while (CONSP (args))
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    {
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      val = eval_sub (XCAR (args));
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      if (NILP (val))
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	break;
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      args = XCDR (args);
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    }

  UNGCPRO;
  return val;
}

DEFUN ("if", Fif, Sif, 2, UNEVALLED, 0,
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       doc: /* If COND yields non-nil, do THEN, else do ELSE...
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Returns the value of THEN or the value of the last of the ELSE's.
THEN must be one expression, but ELSE... can be zero or more expressions.
If COND yields nil, and there are no ELSE's, the value is nil.
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usage: (if COND THEN ELSE...)  */)
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  (Lisp_Object args)
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{
  register Lisp_Object cond;
  struct gcpro gcpro1;

  GCPRO1 (args);
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  cond = eval_sub (Fcar (args));
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  UNGCPRO;

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  if (!NILP (cond))
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    return eval_sub (Fcar (Fcdr (args)));
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  return Fprogn (Fcdr (Fcdr (args)));
}

DEFUN ("cond", Fcond, Scond, 0, UNEVALLED, 0,
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       doc: /* Try each clause until one succeeds.
Each clause looks like (CONDITION BODY...).  CONDITION is evaluated
and, if the value is non-nil, this clause succeeds:
then the expressions in BODY are evaluated and the last one's
value is the value of the cond-form.
If no clause succeeds, cond returns nil.
If a clause has one element, as in (CONDITION),
CONDITION's value if non-nil is returned from the cond-form.
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usage: (cond CLAUSES...)  */)
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  (Lisp_Object args)
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{
  register Lisp_Object clause, val;
  struct gcpro gcpro1;

  val = Qnil;
  GCPRO1 (args);
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  while (!NILP (args))
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    {
      clause = Fcar (args);
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      val = eval_sub (Fcar (clause));
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      if (!NILP (val))
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	{
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	  if (!EQ (XCDR (clause), Qnil))
	    val = Fprogn (XCDR (clause));
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	  break;
	}
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      args = XCDR (args);
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    }
  UNGCPRO;

  return val;
}

DEFUN ("progn", Fprogn, Sprogn, 0, UNEVALLED, 0,
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       doc: /* Eval BODY forms sequentially and return value of last one.
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usage: (progn BODY...)  */)
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  (Lisp_Object args)
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{
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  register Lisp_Object val = Qnil;
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  struct gcpro gcpro1;

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  GCPRO1 (args);
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  while (CONSP (args))
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    {
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      val = eval_sub (XCAR (args));
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      args = XCDR (args);
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    }

  UNGCPRO;
  return val;
}

DEFUN ("prog1", Fprog1, Sprog1, 1, UNEVALLED, 0,
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       doc: /* Eval FIRST and BODY sequentially; return value from FIRST.
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The value of FIRST is saved during the evaluation of the remaining args,
whose values are discarded.
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usage: (prog1 FIRST BODY...)  */)
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  (Lisp_Object args)
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{
  Lisp_Object val;
  register Lisp_Object args_left;
  struct gcpro gcpro1, gcpro2;
  register int argnum = 0;

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  if (NILP (args))
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    return Qnil;

  args_left = args;
  val = Qnil;
  GCPRO2 (args, val);

  do
    {
      if (!(argnum++))
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	val = eval_sub (Fcar (args_left));
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      else
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	eval_sub (Fcar (args_left));
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      args_left = Fcdr (args_left);
    }
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  while (!NILP(args_left));
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  UNGCPRO;
  return val;
}

DEFUN ("prog2", Fprog2, Sprog2, 2, UNEVALLED, 0,
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       doc: /* Eval FORM1, FORM2 and BODY sequentially; return value from FORM2.
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The value of FORM2 is saved during the evaluation of the
remaining args, whose values are discarded.
usage: (prog2 FORM1 FORM2 BODY...)  */)
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  (Lisp_Object args)
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{
  Lisp_Object val;
  register Lisp_Object args_left;
  struct gcpro gcpro1, gcpro2;
  register int argnum = -1;

  val = Qnil;

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  if (NILP (args))
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    return Qnil;

  args_left = args;
  val = Qnil;
  GCPRO2 (args, val);

  do
    {
      if (!(argnum++))
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	val = eval_sub (Fcar (args_left));
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      else
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	eval_sub (Fcar (args_left));
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      args_left = Fcdr (args_left);
    }
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  while (!NILP (args_left));
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  UNGCPRO;
  return val;
}

DEFUN ("setq", Fsetq, Ssetq, 0, UNEVALLED, 0,
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       doc: /* Set each SYM to the value of its VAL.
The symbols SYM are variables; they are literal (not evaluated).
The values VAL are expressions; they are evaluated.
Thus, (setq x (1+ y)) sets `x' to the value of `(1+ y)'.
The second VAL is not computed until after the first SYM is set, and so on;
each VAL can use the new value of variables set earlier in the `setq'.
The return value of the `setq' form is the value of the last VAL.
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usage: (setq [SYM VAL]...)  */)
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  (Lisp_Object args)
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{
  register Lisp_Object args_left;
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  register Lisp_Object val, sym, lex_binding;
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  struct gcpro gcpro1;

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  if (NILP (args))
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    return Qnil;

  args_left = args;
  GCPRO1 (args);

  do
    {
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      val = eval_sub (Fcar (Fcdr (args_left)));
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      sym = Fcar (args_left);
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      /* Like for eval_sub, we do not check declared_special here since
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	 it's been done when let-binding.  */
      if (!NILP (Vinternal_interpreter_environment) /* Mere optimization!  */
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	  && SYMBOLP (sym)
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	  && !NILP (lex_binding
		    = Fassq (sym, Vinternal_interpreter_environment)))
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	XSETCDR (lex_binding, val); /* SYM is lexically bound.  */
      else
	Fset (sym, val);	/* SYM is dynamically bound.  */

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      args_left = Fcdr (Fcdr (args_left));
    }
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  while (!NILP(args_left));
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  UNGCPRO;
  return val;
}
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DEFUN ("quote", Fquote, Squote, 1, UNEVALLED, 0,
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       doc: /* Return the argument, without evaluating it.  `(quote x)' yields `x'.
usage: (quote ARG)  */)
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  (Lisp_Object args)
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{
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  if (!NILP (Fcdr (args)))
    xsignal2 (Qwrong_number_of_arguments, Qquote, Flength (args));
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  return Fcar (args);
}
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DEFUN ("function", Ffunction, Sfunction, 1, UNEVALLED, 0,
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       doc: /* Like `quote', but preferred for objects which are functions.
In byte compilation, `function' causes its argument to be compiled.
`quote' cannot do that.
usage: (function ARG)  */)
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  (Lisp_Object args)
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{
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  Lisp_Object quoted = XCAR (args);

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  if (!NILP (Fcdr (args)))
    xsignal2 (Qwrong_number_of_arguments, Qfunction, Flength (args));
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  if (!NILP (Vinternal_interpreter_environment)
      && CONSP (quoted)
      && EQ (XCAR (quoted), Qlambda))
    /* This is a lambda expression within a lexical environment;
       return an interpreted closure instead of a simple lambda.  */
    return Fcons (Qclosure, Fcons (Vinternal_interpreter_environment, quoted));
  else
    /* Simply quote the argument.  */
    return quoted;
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}

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DEFUN ("interactive-p", Finteractive_p, Sinteractive_p, 0, 0, 0,
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       doc: /* Return t if the containing function was run directly by user input.
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This means that the function was called with `call-interactively'
\(which includes being called as the binding of a key)
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and input is currently coming from the keyboard (not a keyboard macro),
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and Emacs is not running in batch mode (`noninteractive' is nil).

The only known proper use of `interactive-p' is in deciding whether to
display a helpful message, or how to display it.  If you're thinking
of using it for any other purpose, it is quite likely that you're
making a mistake.  Think: what do you want to do when the command is
called from a keyboard macro?

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To test whether your function was called with `call-interactively',
either (i) add an extra optional argument and give it an `interactive'
spec that specifies non-nil unconditionally (such as \"p\"); or (ii)
use `called-interactively-p'.  */)
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  (void)
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{
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  return interactive_p (1) ? Qt : Qnil;
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}


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DEFUN ("called-interactively-p", Fcalled_interactively_p, Scalled_interactively_p, 0, 1, 0,
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       doc: /* Return t if the containing function was called by `call-interactively'.
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If KIND is `interactive', then only return t if the call was made
interactively by the user, i.e. not in `noninteractive' mode nor
when `executing-kbd-macro'.
If KIND is `any', on the other hand, it will return t for any kind of
interactive call, including being called as the binding of a key, or
from a keyboard macro, or in `noninteractive' mode.

The only known proper use of `interactive' for KIND is in deciding
whether to display a helpful message, or how to display it.  If you're
thinking of using it for any other purpose, it is quite likely that
you're making a mistake.  Think: what do you want to do when the
command is called from a keyboard macro?
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This function is meant for implementing advice and other
function-modifying features.  Instead of using this, it is sometimes
cleaner to give your function an extra optional argument whose
`interactive' spec specifies non-nil unconditionally (\"p\" is a good
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way to do this), or via (not (or executing-kbd-macro noninteractive)).  */)
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  (Lisp_Object kind)
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{
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  return ((INTERACTIVE || !EQ (kind, intern ("interactive")))
	  && interactive_p (1)) ? Qt : Qnil;
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}


/*  Return 1 if function in which this appears was called using
    call-interactively.
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    EXCLUDE_SUBRS_P non-zero means always return 0 if the function
    called is a built-in.  */

int
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interactive_p (int exclude_subrs_p)
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{
  struct backtrace *btp;
  Lisp_Object fun;
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  btp = backtrace_list;
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  /* If this isn't a byte-compiled function, there may be a frame at
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     the top for Finteractive_p.  If so, skip it.  */
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  fun = Findirect_function (*btp->function, Qnil);
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  if (SUBRP (fun) && (XSUBR (fun) == &Sinteractive_p
		      || XSUBR (fun) == &Scalled_interactively_p))
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    btp = btp->next;
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  /* If we're running an Emacs 18-style byte-compiled function, there
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     may be a frame for Fbytecode at the top level.  In any version of
     Emacs there can be Fbytecode frames for subexpressions evaluated
     inside catch and condition-case.  Skip past them.
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     If this isn't a byte-compiled function, then we may now be
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     looking at several frames for special forms.  Skip past them.  */
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  while (btp
	 && (EQ (*btp->function, Qbytecode)
	     || btp->nargs == UNEVALLED))
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    btp = btp->next;

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  /* btp now points at the frame of the innermost function that isn't
     a special form, ignoring frames for Finteractive_p and/or
     Fbytecode at the top.  If this frame is for a built-in function
     (such as load or eval-region) return nil.  */
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  fun = Findirect_function (*btp->function, Qnil);
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  if (exclude_subrs_p && SUBRP (fun))
    return 0;
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  /* btp points to the frame of a Lisp function that called interactive-p.
     Return t if that function was called interactively.  */
  if (btp && btp->next && EQ (*btp->next->function, Qcall_interactively))
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    return 1;
  return 0;
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}

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DEFUN ("defun", Fdefun, Sdefun, 2, UNEVALLED, 0,
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       doc: /* Define NAME as a function.
The definition is (lambda ARGLIST [DOCSTRING] BODY...).
See also the function `interactive'.
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usage: (defun NAME ARGLIST [DOCSTRING] BODY...)  */)
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  (Lisp_Object args)
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{
  register Lisp_Object fn_name;
  register Lisp_Object defn;

  fn_name = Fcar (args);
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  CHECK_SYMBOL (fn_name);
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  defn = Fcons (Qlambda, Fcdr (args));
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  if (!NILP (Vinternal_interpreter_environment)) /* Mere optimization!  */
    defn = Ffunction (Fcons (defn, Qnil));
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  if (!NILP (Vpurify_flag))
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    defn = Fpurecopy (defn);
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  if (CONSP (XSYMBOL (fn_name)->function)
      && EQ (XCAR (XSYMBOL (fn_name)->function), Qautoload))
    LOADHIST_ATTACH (Fcons (Qt, fn_name));
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  Ffset (fn_name, defn);
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  LOADHIST_ATTACH (Fcons (Qdefun, fn_name));
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  return fn_name;
}

DEFUN ("defmacro", Fdefmacro, Sdefmacro, 2, UNEVALLED, 0,
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       doc: /* Define NAME as a macro.
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The actual definition looks like
 (macro lambda ARGLIST [DOCSTRING] [DECL] BODY...).
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When the macro is called, as in (NAME ARGS...),
the function (lambda ARGLIST BODY...) is applied to
the list ARGS... as it appears in the expression,
and the result should be a form to be evaluated instead of the original.
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DECL is a declaration, optional, which can specify how to indent
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calls to this macro, how Edebug should handle it, and which argument
should be treated as documentation.  It looks like this:
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  (declare SPECS...)
The elements can look like this:
  (indent INDENT)
	Set NAME's `lisp-indent-function' property to INDENT.

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  (debug DEBUG)
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	Set NAME's `edebug-form-spec' property to DEBUG.  (This is
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	equivalent to writing a `def-edebug-spec' for the macro.)
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  (doc-string ELT)
	Set NAME's `doc-string-elt' property to ELT.

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usage: (defmacro NAME ARGLIST [DOCSTRING] [DECL] BODY...)  */)
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  (Lisp_Object args)
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{
  register Lisp_Object fn_name;
  register Lisp_Object defn;
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  Lisp_Object lambda_list, doc, tail;
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  fn_name = Fcar (args);
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  CHECK_SYMBOL (fn_name);
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  lambda_list = Fcar (Fcdr (args));
  tail = Fcdr (Fcdr (args));

  doc = Qnil;
  if (STRINGP (Fcar (tail)))
    {
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      doc = XCAR (tail);
      tail = XCDR (tail);
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    }

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  if (CONSP (Fcar (tail))
      && EQ (Fcar (Fcar (tail)), Qdeclare))
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    {
      if (!NILP (Vmacro_declaration_function))
	{
	  struct gcpro gcpro1;
	  GCPRO1 (args);
	  call2 (Vmacro_declaration_function, fn_name, Fcar (tail));
	  UNGCPRO;
	}
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      tail = Fcdr (tail);
    }

  if (NILP (doc))
    tail = Fcons (lambda_list, tail);
  else
    tail = Fcons (lambda_list, Fcons (doc, tail));
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  defn = Fcons (Qlambda, tail);
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  if (!NILP (Vinternal_interpreter_environment)) /* Mere optimization!  */
    defn = Ffunction (Fcons (defn, Qnil));
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  defn = Fcons (Qmacro, defn);
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  if (!NILP (Vpurify_flag))
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    defn = Fpurecopy (defn);
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  if (CONSP (XSYMBOL (fn_name)->function)
      && EQ (XCAR (XSYMBOL (fn_name)->function), Qautoload))
    LOADHIST_ATTACH (Fcons (Qt, fn_name));
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  Ffset (fn_name, defn);
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  LOADHIST_ATTACH (Fcons (Qdefun, fn_name));
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  return fn_name;
}

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DEFUN ("defvaralias", Fdefvaralias, Sdefvaralias, 2, 3, 0,
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       doc: /* Make NEW-ALIAS a variable alias for symbol BASE-VARIABLE.
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Aliased variables always have the same value; setting one sets the other.
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Third arg DOCSTRING, if non-nil, is documentation for NEW-ALIAS.  If it is
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omitted or nil, NEW-ALIAS gets the documentation string of BASE-VARIABLE,
or of the variable at the end of the chain of aliases, if BASE-VARIABLE is
itself an alias.  If NEW-ALIAS is bound, and BASE-VARIABLE is not,
then the value of BASE-VARIABLE is set to that of NEW-ALIAS.
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The return value is BASE-VARIABLE.  */)
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  (Lisp_Object new_alias, Lisp_Object base_variable, Lisp_Object docstring)
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{
  struct Lisp_Symbol *sym;
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  CHECK_SYMBOL (new_alias);
  CHECK_SYMBOL (base_variable);
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  sym = XSYMBOL (new_alias);
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  if (sym->constant)
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    /* Not sure why, but why not?  */
    error ("Cannot make a constant an alias");
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  switch (sym->redirect)
    {
    case SYMBOL_FORWARDED:
      error ("Cannot make an internal variable an alias");
    case SYMBOL_LOCALIZED:
      error ("Don't know how to make a localized variable an alias");
    }

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  /* http://lists.gnu.org/archive/html/emacs-devel/2008-04/msg00834.html
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     If n_a is bound, but b_v is not, set the value of b_v to n_a,
     so that old-code that affects n_a before the aliasing is setup
     still works.  */
  if (NILP (Fboundp (base_variable)))
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    set_internal (base_variable, find_symbol_value (new_alias), Qnil, 1);
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  {
    struct specbinding *p;

    for (p = specpdl_ptr - 1; p >= specpdl; p--)
      if (p->func == NULL
	  && (EQ (new_alias,
		  CONSP (p->symbol) ? XCAR (p->symbol) : p->symbol)))
	error ("Don't know how to make a let-bound variable an alias");
  }

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  sym->declared_special = 1;
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  sym->redirect = SYMBOL_VARALIAS;
  SET_SYMBOL_ALIAS (sym, XSYMBOL (base_variable));
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  sym->constant = SYMBOL_CONSTANT_P (base_variable);
  LOADHIST_ATTACH (new_alias);
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  /* Even if docstring is nil: remove old docstring.  */
  Fput (new_alias, Qvariable_documentation, docstring);
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  return base_variable;
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}


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DEFUN ("defvar", Fdefvar, Sdefvar, 1, UNEVALLED, 0,
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       doc: /* Define SYMBOL as a variable, and return SYMBOL.
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You are not required to define a variable in order to use it,
but the definition can supply documentation and an initial value
in a way that tags can recognize.

INITVALUE is evaluated, and used to set SYMBOL, only if SYMBOL's value is void.
If SYMBOL is buffer-local, its default value is what is set;
 buffer-local values are not affected.
INITVALUE and DOCSTRING are optional.
If DOCSTRING starts with *, this variable is identified as a user option.
 This means that M-x set-variable recognizes it.
 See also `user-variable-p'.
If INITVALUE is missing, SYMBOL's value is not set.
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If SYMBOL has a local binding, then this form affects the local
binding.  This is usually not what you want.  Thus, if you need to
load a file defining variables, with this form or with `defconst' or
`defcustom', you should always load that file _outside_ any bindings
for these variables.  \(`defconst' and `defcustom' behave similarly in
this respect.)
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usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
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  (Lisp_Object args)
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{
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  register Lisp_Object sym, tem, tail;
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  sym = Fcar (args);
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  tail = Fcdr (args);
  if (!NILP (Fcdr (Fcdr (tail))))
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    error ("Too many arguments");
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  tem = Fdefault_boundp (sym);
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  if (!NILP (tail))
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    {
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