eval.c 120 KB
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/* Evaluator for GNU Emacs Lisp interpreter.
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Copyright (C) 1985-1987, 1993-1995, 1999-2016 Free Software Foundation,
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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 <stdlib.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 "buffer.h"
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/* Chain of condition and catch handlers currently in effect.  */
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/* struct handler *handlerlist; */
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/* 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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/* This holds either the symbol `run-hooks' or nil.
   It is nil at an early stage of startup, and when Emacs
   is shutting down.  */
Lisp_Object Vrun_hooks;

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/* The commented-out variables below are macros defined in thread.h.  */

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/* Current number of specbindings allocated in specpdl, not counting
   the dummy entry specpdl[-1].  */
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/* ptrdiff_t specpdl_size; */
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/* Pointer to beginning of specpdl.  A dummy entry specpdl[-1] exists
   only so that its address can be taken.  */
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/* union specbinding *specpdl; */
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/* Pointer to first unused element in specpdl.  */
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/* union 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 unused.  */
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Lisp_Object inhibit_lisp_code;

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/* These would ordinarily be static, but they need to be visible to GDB.  */
bool backtrace_p (union specbinding *) EXTERNALLY_VISIBLE;
Lisp_Object *backtrace_args (union specbinding *) EXTERNALLY_VISIBLE;
Lisp_Object backtrace_function (union specbinding *) EXTERNALLY_VISIBLE;
union specbinding *backtrace_next (union specbinding *) EXTERNALLY_VISIBLE;
union specbinding *backtrace_top (void) EXTERNALLY_VISIBLE;

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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, Lisp_Object, ptrdiff_t);
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static Lisp_Object lambda_arity (Lisp_Object);
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static Lisp_Object
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specpdl_symbol (union specbinding *pdl)
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{
  eassert (pdl->kind >= SPECPDL_LET);
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  return pdl->let.symbol;
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}

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static enum specbind_tag
specpdl_kind (union specbinding *pdl)
{
  eassert (pdl->kind >= SPECPDL_LET);
  return pdl->let.kind;
}

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static Lisp_Object
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specpdl_old_value (union specbinding *pdl)
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{
  eassert (pdl->kind >= SPECPDL_LET);
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  return pdl->let.old_value;
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}

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static void
set_specpdl_old_value (union specbinding *pdl, Lisp_Object val)
{
  eassert (pdl->kind >= SPECPDL_LET);
  pdl->let.old_value = val;
}

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static Lisp_Object
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specpdl_where (union specbinding *pdl)
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{
  eassert (pdl->kind > SPECPDL_LET);
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  return pdl->let.where;
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}

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static Lisp_Object
specpdl_saved_value (union specbinding *pdl)
{
  eassert (pdl->kind >= SPECPDL_LET);
  return pdl->let.saved_value;
}

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static Lisp_Object
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specpdl_arg (union specbinding *pdl)
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{
  eassert (pdl->kind == SPECPDL_UNWIND);
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  return pdl->unwind.arg;
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}

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Lisp_Object
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backtrace_function (union specbinding *pdl)
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{
  eassert (pdl->kind == SPECPDL_BACKTRACE);
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  return pdl->bt.function;
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}

static ptrdiff_t
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backtrace_nargs (union specbinding *pdl)
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{
  eassert (pdl->kind == SPECPDL_BACKTRACE);
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  return pdl->bt.nargs;
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}

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Lisp_Object *
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backtrace_args (union specbinding *pdl)
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{
  eassert (pdl->kind == SPECPDL_BACKTRACE);
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  return pdl->bt.args;
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}

static bool
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backtrace_debug_on_exit (union specbinding *pdl)
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{
  eassert (pdl->kind == SPECPDL_BACKTRACE);
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  return pdl->bt.debug_on_exit;
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}

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/* Functions to modify slots of backtrace records.  */
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static void
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set_backtrace_args (union specbinding *pdl, Lisp_Object *args, ptrdiff_t nargs)
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{
  eassert (pdl->kind == SPECPDL_BACKTRACE);
  pdl->bt.args = args;
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  pdl->bt.nargs = nargs;
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}
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static void
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set_backtrace_debug_on_exit (union specbinding *pdl, bool doe)
{
  eassert (pdl->kind == SPECPDL_BACKTRACE);
  pdl->bt.debug_on_exit = doe;
}
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/* Helper functions to scan the backtrace.  */

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bool
backtrace_p (union specbinding *pdl)
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{ return pdl >= specpdl; }
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union specbinding *
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backtrace_top (void)
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{
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  union 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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union specbinding *
backtrace_next (union 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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/* Return a pointer to somewhere near the top of the C stack.  */
void *
near_C_stack_top (void)
{
  return backtrace_args (backtrace_top ());
}
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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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  union specbinding *pdlvec = xmalloc ((size + 1) * sizeof *specpdl);
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  specpdl_size = size;
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  specpdl = specpdl_ptr = pdlvec + 1;
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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 = 800;
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  Vrun_hooks = Qnil;
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}

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/* static struct handler handlerlist_sentinel; */
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void
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init_eval (void)
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{
  specpdl_ptr = specpdl;
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  { /* Put a dummy catcher at top-level so that handlerlist is never NULL.
       This is important since handlerlist->nextfree holds the freelist
       which would otherwise leak every time we unwind back to top-level.   */
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    handlerlist_sentinel = xzalloc (sizeof (struct handler));
    handlerlist = handlerlist_sentinel->nextfree = handlerlist_sentinel;
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    struct handler *c = push_handler (Qunbound, CATCHER);
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    eassert (c == handlerlist_sentinel);
    handlerlist_sentinel->nextfree = NULL;
    handlerlist_sentinel->next = NULL;
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  }
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  Vquit_flag = Qnil;
  debug_on_next_call = 0;
  lisp_eval_depth = 0;
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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 void
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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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static void grow_specpdl (void);

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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_depth = max_lisp_eval_depth;
  /* Do not allow max_specpdl_size less than actual depth (Bug#16603).  */
  EMACS_INT old_max = max (max_specpdl_size, count);
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  if (lisp_eval_depth + 40 > max_lisp_eval_depth)
    max_lisp_eval_depth = lisp_eval_depth + 40;

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  /* While debugging Bug#16603, previous value of 100 was found
     too small to avoid specpdl overflow in the debugger itself.  */
  if (max_specpdl_size - 200 < count)
    max_specpdl_size = count + 200;

  if (old_max == count)
    {
      /* We can enter the debugger due to specpdl overflow (Bug#16603).  */
      specpdl_ptr--;
      grow_specpdl ();
    }

  /* Restore limits after leaving the debugger.  */
  record_unwind_protect (restore_stack_limits,
			 Fcons (make_number (old_max),
				make_number (old_depth)));
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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 we are debugging an error while `inhibit-changing-match-data'
     is bound to non-nil (e.g., within a call to `string-match-p'),
     then make sure debugger code can still use match data.  */
  specbind (Qinhibit_changing_match_data, 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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static void
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do_debug_on_call (Lisp_Object code, ptrdiff_t count)
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{
  debug_on_next_call = 0;
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  set_backtrace_debug_on_exit (specpdl + count, true);
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  call_debugger (list1 (code));
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}

/* 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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  Lisp_Object val = Qnil;
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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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    }

  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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  Lisp_Object val = Qt;
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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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    }

  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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{
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  Lisp_Object cond;
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  cond = eval_sub (XCAR (args));
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  if (!NILP (cond))
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    return eval_sub (Fcar (XCDR (args)));
  return Fprogn (XCDR (XCDR (args)));
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}

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.
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If a clause has one element, as in (CONDITION), then the cond-form
returns CONDITION's value, if that is non-nil.
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If no clause succeeds, cond returns nil.
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usage: (cond CLAUSES...)  */)
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  (Lisp_Object args)
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{
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  Lisp_Object val = args;
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  while (CONSP (args))
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    {
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      Lisp_Object clause = XCAR (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 (!NILP (XCDR (clause)))
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	    val = Fprogn (XCDR (clause));
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	  break;
	}
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      args = XCDR (args);
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    }

  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 body)
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{
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  Lisp_Object val = Qnil;
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  while (CONSP (body))
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    {
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      val = eval_sub (XCAR (body));
      body = XCDR (body);
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    }

  return val;
}

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/* Evaluate BODY sequentially, discarding its value.  Suitable for
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   record_unwind_protect.  */

void
unwind_body (Lisp_Object body)
{
  Fprogn (body);
}

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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;
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  Lisp_Object args_left;
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  args_left = args;
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  val = args;
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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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  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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  eval_sub (XCAR (args));
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  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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{
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  Lisp_Object val, sym, lex_binding;
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  val = args;
  if (CONSP (args))
    {
      Lisp_Object args_left = args;
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      Lisp_Object numargs = Flength (args);

      if (XINT (numargs) & 1)
        xsignal2 (Qwrong_number_of_arguments, Qsetq, numargs);
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      do
	{
	  val = eval_sub (Fcar (XCDR (args_left)));
	  sym = XCAR (args_left);

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

  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').
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This means that \\='(a . b) is not identical to (cons \\='a \\='b): the former
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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 (CONSP (XCDR (args)))
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    xsignal2 (Qwrong_number_of_arguments, Qquote, Flength (args));
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  return XCAR (args);
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}
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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 (CONSP (XCDR (args)))
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    xsignal2 (Qwrong_number_of_arguments, Qfunction, Flength (args));
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  if (!NILP (Vinternal_interpreter_environment)
      && CONSP (quoted)
      && EQ (XCAR (quoted), Qlambda))
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    { /* This is a lambda expression within a lexical environment;
	 return an interpreted closure instead of a simple lambda.  */
      Lisp_Object cdr = XCDR (quoted);
      Lisp_Object tmp = cdr;
      if (CONSP (tmp)
	  && (tmp = XCDR (tmp), CONSP (tmp))
	  && (tmp = XCAR (tmp), CONSP (tmp))
	  && (EQ (QCdocumentation, XCAR (tmp))))
	{ /* Handle the special (:documentation <form>) to build the docstring
	     dynamically.  */
	  Lisp_Object docstring = eval_sub (Fcar (XCDR (tmp)));
	  CHECK_STRING (docstring);
	  cdr = Fcons (XCAR (cdr), Fcons (docstring, XCDR (XCDR (cdr))));
	}
      return Fcons (Qclosure, Fcons (Vinternal_interpreter_environment,
				     cdr));
    }
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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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  if (SYMBOL_CONSTANT_P (new_alias))
    /* Making it an alias effectively changes its value.  */
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    error ("Cannot make a constant an alias");
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  sym = XSYMBOL (new_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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    case SYMBOL_PLAINVAL:
    case SYMBOL_VARALIAS:
      break;
    default:
      emacs_abort ();
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    }

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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, SET_INTERNAL_BIND);
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  {
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    union 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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  if (sym->trapped_write == SYMBOL_TRAPPED_WRITE)
    notify_variable_watchers (new_alias, base_variable, Qdefvaralias, Qnil);

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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->trapped_write = XSYMBOL (base_variable)->trapped_write;
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  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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static union specbinding *
default_toplevel_binding (Lisp_Object symbol)
{
  union specbinding *binding = NULL;
  union specbinding *pdl = specpdl_ptr;
  while (pdl > specpdl)
    {
      switch ((--pdl)->kind)
	{
	case SPECPDL_LET_DEFAULT:
	case SPECPDL_LET:
	  if (EQ (specpdl_symbol (pdl), symbol))
	    binding = pdl;
	  break;
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	case SPECPDL_UNWIND:
	case SPECPDL_UNWIND_PTR:
	case SPECPDL_UNWIND_INT:
	case SPECPDL_UNWIND_VOID:
	case SPECPDL_BACKTRACE:
	case SPECPDL_LET_LOCAL:
	  break;

	default:
	  emacs_abort ();
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	}
    }
  return binding;
}

DEFUN ("default-toplevel-value", Fdefault_toplevel_value, Sdefault_toplevel_value, 1, 1, 0,
       doc: /* Return SYMBOL's toplevel default value.
"Toplevel" means outside of any let binding.  */)
  (Lisp_Object symbol)
{
  union specbinding *binding = default_toplevel_binding (symbol);
  Lisp_Object value
    = binding ? specpdl_old_value (binding) : Fdefault_value (symbol);
  if (!EQ (value, Qunbound))
    return value;
  xsignal1 (Qvoid_variable, symbol);
}

DEFUN ("set-default-toplevel-value", Fset_default_toplevel_value,
       Sset_default_toplevel_value, 2, 2, 0,
       doc: /* Set SYMBOL's toplevel default value to VALUE.
"Toplevel" means outside of any let binding.  */)
     (Lisp_Object symbol, Lisp_Object value)
{
  union specbinding *binding = default_toplevel_binding (symbol);
  if (binding)
    set_specpdl_old_value (binding, value);
  else
    Fset_default (symbol, value);
  return Qnil;
}
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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
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for these variables.  (`defconst' and `defcustom' behave similarly in
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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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  Lisp_Object sym, tem, tail;
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  sym = XCAR (args);
  tail = XCDR (args);
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  if (CONSP (tail))
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    {
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      if (CONSP (XCDR (tail)) && CONSP (XCDR (XCDR (tail))))
	error ("Too many arguments");

      tem = Fdefault_boundp (sym);

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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 (XCAR (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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	  union specbinding *binding = default_toplevel_binding (sym);
	  if (binding && EQ (specpdl_old_value (binding), Qunbound))
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	    {
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	      set_specpdl_old_value (binding, eval_sub (XCAR (tail)));
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	    }
	}
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      tail = XCDR (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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{
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  Lisp_Object sym, tem;
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  sym = XCAR (args);
  if (CONSP (Fcdr (XCDR (XCDR (args)))))
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    error ("Too many arguments");
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  tem = eval_sub (Fcar (XCDR (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 (XCDR (XCDR (args)));