eval.c 101 KB
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/* Evaluator for GNU Emacs Lisp interpreter.
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   Copyright (C) 1985-1987, 1993-1995, 1999-2013 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 <limits.h>
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#include <stdio.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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#if !BYTE_MARK_STACK
static
#endif
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struct catchtag *catchlist;

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/* Chain of condition handlers currently in effect.
   The elements of this chain are contained in the stack frames
   of Fcondition_case and internal_condition_case.
   When an error is signaled (by calling Fsignal, below),
   this chain is searched for an element that applies.  */

#if !BYTE_MARK_STACK
static
#endif
struct handler *handlerlist;

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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;
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Lisp_Object Qinhibit_quit;
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Lisp_Object Qand_rest;
static Lisp_Object Qand_optional;
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static Lisp_Object Qinhibit_debugger;
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static Lisp_Object Qdeclare;
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Lisp_Object Qinternal_interpreter_environment, Qclosure;

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static 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;

/* Current number of specbindings allocated in specpdl.  */
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ptrdiff_t 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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/* Depth in Lisp evaluations and function calls.  */
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static EMACS_INT lisp_eval_depth;
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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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static EMACS_INT when_entered_debugger;
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/* The function from which the last `signal' was called.  Set in
   Fsignal.  */
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/* FIXME: We should probably get rid of this!  */
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Lisp_Object Vsignaling_function;

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/* If non-nil, Lisp code must not be run since some part of Emacs is
   in an inconsistent state.  Currently, x-create-frame uses this to
   avoid triggering window-configuration-change-hook while the new
   frame is half-initialized.  */
Lisp_Object inhibit_lisp_code;

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static Lisp_Object funcall_lambda (Lisp_Object, ptrdiff_t, Lisp_Object *);
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static Lisp_Object apply_lambda (Lisp_Object fun, Lisp_Object args);
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/* Functions to modify slots of backtrace records.  */
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static void
set_backtrace_args (struct specbinding *pdl, Lisp_Object *args)
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{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.args = args; }

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static void
set_backtrace_nargs (struct specbinding *pdl, ptrdiff_t n)
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{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.nargs = n; }

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static void
set_backtrace_debug_on_exit (struct specbinding *pdl, bool doe)
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{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.debug_on_exit = doe; }

/* Helper functions to scan the backtrace.  */

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bool backtrace_p (struct specbinding *) EXTERNALLY_VISIBLE;
struct specbinding *backtrace_top (void) EXTERNALLY_VISIBLE;
struct specbinding *backtrace_next (struct specbinding *pdl) EXTERNALLY_VISIBLE;

bool backtrace_p (struct specbinding *pdl)
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{ return pdl >= specpdl; }
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struct specbinding *
backtrace_top (void)
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{
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  struct specbinding *pdl = specpdl_ptr - 1;
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  while (backtrace_p (pdl) && pdl->kind != SPECPDL_BACKTRACE)
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    pdl--;
  return pdl;
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}
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struct specbinding *
backtrace_next (struct specbinding *pdl)
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{
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  pdl--;
  while (backtrace_p (pdl) && pdl->kind != SPECPDL_BACKTRACE)
    pdl--;
  return pdl;
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}

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void
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init_eval_once (void)
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{
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  enum { size = 50 };
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  specpdl = xmalloc (size * sizeof *specpdl);
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  specpdl_size = size;
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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 = 1300; /* 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;
  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.  */
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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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  bool debug_while_redisplaying;
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  ptrdiff_t 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;

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  if (max_specpdl_size - 100 < SPECPDL_INDEX ())
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    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 (Qinhibit_debugger, Qt);
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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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static void
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do_debug_on_call (Lisp_Object code)
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{
  debug_on_next_call = 0;
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  set_backtrace_debug_on_exit (specpdl_ptr - 1, true);
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  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;
}

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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;

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

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  val = eval_sub (XCAR (args_left));
  while (CONSP (args_left = XCDR (args_left)))
    eval_sub (XCAR (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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{
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  struct gcpro gcpro1;
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  GCPRO1 (args);
  eval_sub (XCAR (args));
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  UNGCPRO;
  return Fprog1 (XCDR (args));
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}

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'.
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Warning: `quote' does not construct its return value, but just returns
the value that was pre-constructed by the Lisp reader (see info node
`(elisp)Printed Representation').
This means that '(a . b) is not identical to (cons 'a 'b): the former
does not cons.  Quoting should be reserved for constants that will
never be modified by side-effects, unless you like self-modifying code.
See the common pitfall in info node `(elisp)Rearrangement' for an example
of unexpected results when a quoted object is modified.
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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.  */
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    return Fcons (Qclosure, Fcons (Vinternal_interpreter_environment,
				   XCDR (quoted)));
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  else
    /* Simply quote the argument.  */
    return quoted;
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}

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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;

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    for (p = specpdl_ptr; p > specpdl; )
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      if ((--p)->kind >= SPECPDL_LET
	  && (EQ (new_alias, specpdl_symbol (p))))
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	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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  XSYMBOL (base_variable)->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
defining it lets you supply an initial value and documentation, which
can be referred to by the Emacs help facilities and other programming
tools.  The `defvar' form also declares the variable as \"special\",
so that it is always dynamically bound even if `lexical-binding' is t.

The optional argument 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.
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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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The optional argument DOCSTRING is a documentation string for the
variable.

To define a user option, use `defcustom' instead of `defvar'.
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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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      /* Do it before evaluating the initial value, for self-references.  */
      XSYMBOL (sym)->declared_special = 1;
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      if (NILP (tem))
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	Fset_default (sym, eval_sub (Fcar (tail)));
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      else
	{ /* Check if there is really a global binding rather than just a let
	     binding that shadows the global unboundness of the var.  */
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	  struct specbinding *pdl = specpdl_ptr;
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	  while (pdl > specpdl)
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	    {
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	      if ((--pdl)->kind >= SPECPDL_LET
		  && EQ (specpdl_symbol (pdl), sym)
		  && EQ (specpdl_old_value (pdl), Qunbound))
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		{
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		  message_with_string
		    ("Warning: defvar ignored because %s is let-bound",
		     SYMBOL_NAME (sym), 1);
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		  break;
		}
	    }
	}
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      tail = Fcdr (tail);
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      tem = Fcar (tail);
      if (!NILP (tem))
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	{
	  if (!NILP (Vpurify_flag))
	    tem = Fpurecopy (tem);
	  Fput (sym, Qvariable_documentation, tem);
	}
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      LOADHIST_ATTACH (sym);
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    }
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  else if (!NILP (Vinternal_interpreter_environment)
	   && !XSYMBOL (sym)->declared_special)
    /* A simple (defvar foo) with lexical scoping does "nothing" except
       declare that var to be dynamically scoped *locally* (i.e. within
       the current file or let-block).  */
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    Vinternal_interpreter_environment
      = Fcons (sym, Vinternal_interpreter_environment);
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  else
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    {
      /* Simple (defvar <var>) should not count as a definition at all.
	 It could get in the way of other definitions, and unloading this
	 package could try to make the variable unbound.  */
    }
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  return sym;
}

DEFUN ("defconst", Fdefconst, Sdefconst, 2, UNEVALLED, 0,
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       doc: /* Define SYMBOL as a constant variable.
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This declares that neither programs nor users should ever change the
value.  This constancy is not actually enforced by Emacs Lisp, but
SYMBOL is marked as a special variable so that it is never lexically
bound.

The `defconst' form always sets the value of SYMBOL to the result of
evalling INITVALUE.  If SYMBOL is buffer-local, its default value is
what is set; buffer-local values are not affected.  If SYMBOL has a
local binding, then this form sets the local binding's value.
However, you should normally not make local bindings for variables
defined with this form.

The optional DOCSTRING specifies the variable's documentation string.
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usage: (defconst SYMBOL INITVALUE [DOCSTRING])  */)
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  (Lisp_Object args)
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{
  register Lisp_Object sym, tem;

  sym = Fcar (args);
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  if (!NILP (Fcdr (Fcdr (Fcdr (args)))))
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    error ("Too many arguments");
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  tem = eval_sub (Fcar (Fcdr (args)));
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  if (!NILP (Vpurify_flag))
    tem = Fpurecopy (tem);
  Fset_default (sym, tem);
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  XSYMBOL (sym)->declared_special = 1;
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  tem = Fcar (Fcdr (Fcdr (args)));
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  if (!NILP (tem))
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    {
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      if (!NILP (Vpurify_flag))
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	tem = Fpurecopy (tem);
      Fput (sym, Qvariable_documentation, tem);
    }
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  Fput (sym, Qrisky_local_variable, Qt);
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  LOADHIST_ATTACH (sym);
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  return sym;
}

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/* Make SYMBOL lexically scoped.  */
DEFUN ("internal-make-var-non-special", Fmake_var_non_special,
       Smake_var_non_special, 1, 1, 0,
       doc: /* Internal function.  */)
     (Lisp_Object symbol)
{
  CHECK_SYMBOL (symbol);
  XSYMBOL (symbol)->declared_special = 0;
  return Qnil;
}

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DEFUN ("let*", FletX, SletX, 1, UNEVALLED, 0,
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       doc: /* Bind variables according to VARLIST then eval BODY.
The value of the last form in BODY is returned.
Each element of VARLIST is a symbol (which is bound to nil)
or a list (SYMBOL VALUEFORM) (which binds SYMBOL to the value of VALUEFORM).
Each VALUEFORM can refer to the symbols already bound by this VARLIST.
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usage: (let* VARLIST BODY...)  */)
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  (Lisp_Object args)
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{
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  Lisp_Object varlist, var, val, elt, lexenv;
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  ptrdiff_t count = SPECPDL_INDEX ();
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  struct gcpro gcpro1, gcpro2, gcpro3;

  GCPRO3 (args, elt, varlist);

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  lexenv = Vinternal_interpreter_environment;

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  varlist = Fcar (args);
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  while (CONSP (varlist))
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    {
      QUIT;
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      elt = XCAR (varlist);
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      if (SYMBOLP (elt))
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	{
	  var = elt;
	  val = Qnil;
	}
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      else if (! NILP (Fcdr (Fcdr (elt))))
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	signal_error ("`let' bindings can have only one value-form", elt);
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      else
	{
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	  var = Fcar (elt);
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	  val = eval_sub (Fcar (Fcdr (elt)));
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	}
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      if (!NILP (lexenv) && SYMBOLP (var)
	  && !XSYMBOL (var)->declared_special
	  && NILP (Fmemq (var, Vinternal_interpreter_environment)))
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	/* Lexically bind VAR by adding it to the interpreter's binding
	   alist.  */
	{
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	  Lisp_Object newenv
	    = Fcons (Fcons (var, val), Vinternal_interpreter_environment);
	  if (EQ (Vinternal_interpreter_environment, lexenv))
	    /* Save the old lexical environment on the specpdl stack,
	       but only for the first lexical binding, since we'll never
	       need to revert to one of the intermediate ones.  */
	    specbind (Qinternal_interpreter_environment, newenv);
	  else
	    Vinternal_interpreter_environment = newenv;
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	}
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      else
	specbind (var, val);

      varlist = XCDR (varlist);
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    }
  UNGCPRO;
  val = Fprogn (Fcdr (args));
  return unbind_to (count, val);
}

DEFUN ("let", Flet, Slet, 1, UNEVALLED, 0,
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       doc: /* Bind variables according to VARLIST then eval BODY.
The value of the last form in BODY is returned.
Each element of VARLIST is a symbol (which is bound to nil)
or a list (SYMBOL VALUEFORM) (which binds SYMBOL to the value of VALUEFORM).
All the VALUEFORMs are evalled before any symbols are bound.
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usage: (let VARLIST BODY...)  */)
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  (Lisp_Object args)
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{
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  Lisp_Object *temps, tem, lexenv;
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  register Lisp_Object elt, varlist;
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  ptrdiff_t count = SPECPDL_INDEX ();
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  ptrdiff_t argnum;
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  struct gcpro gcpro1, gcpro2;
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  USE_SAFE_ALLOCA;
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  varlist = Fcar (args);

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  /* Make space to hold the values to give the bound variables.  */
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  elt = Flength (varlist);
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  SAFE_ALLOCA_LISP (temps, XFASTINT (elt));
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  /* Compute the values and store them in `temps'.  */
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  GCPRO2 (args, *temps);
  gcpro2.nvars = 0;

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  for (argnum = 0; CONSP (varlist); varlist = XCDR (varlist))
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    {
      QUIT;
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      elt = XCAR (varlist);
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      if (SYMBOLP (elt))
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	temps [argnum++] = Qnil;
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      else if (! NILP (Fcdr (Fcdr (elt))))
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	signal_error ("`let' bindings can have only one value-form", elt);
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      else
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	temps [argnum++] = eval_sub (Fcar (Fcdr (elt)));
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      gcpro2.nvars = argnum;
    }
  UNGCPRO;

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  lexenv = Vinternal_interpreter_environment;

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  varlist = Fcar (args);
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  for (argnum = 0; CONSP (varlist); varlist = XCDR (varlist))
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    {
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      Lisp_Object var;

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      elt = XCAR (varlist);
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