This text is extracted from the file camlp4/Camlp4Parsers/Camlp4MacroParser.ml
of OCaml 3.12.0 release.

Only copyright and useful comments are kept.

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(*                                                                          *)
(*                              Objective Caml                              *)
(*                                                                          *)
(*                            INRIA Rocquencourt                            *)
(*                                                                          *)
(*  Copyright  2006   Institut National de Recherche  en  Informatique et   *)
(*  en Automatique.  All rights reserved.  This file is distributed under   *)
(*  the terms of the GNU Library General Public License, with the special   *)
(*  exception on linking described in LICENSE at the top of the Objective   *)
(*  Caml source tree.                                                       *)
(*                                                                          *)
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(* Authors:
 * - Daniel de Rauglaudre: initial version
 * - Nicolas Pouillard: refactoring
 * - Aleksey Nogin: extra features and bug fixes.
 * - Christopher Conway: extra feature (-D<uident>=)
 * - Jean-vincent Loddo: definitions inside IFs.
 *)

(*
Added statements:

  At toplevel (structure item):

     DEFINE <uident>
     DEFINE <uident> = <expression>
     DEFINE <uident> (<parameters>) = <expression>
     IFDEF <uident> THEN <structure_items> [ ELSE <structure_items> ] (END | ENDIF)
     IFNDEF <uident> THEN <structure_items> [ ELSE <structure_items> ] (END | ENDIF)
     INCLUDE <string>

  At toplevel (signature item):

     DEFINE <uident>
     IFDEF <uident> THEN <signature_items> [ ELSE <signature_items> ] (END | ENDIF)
     IFNDEF <uident> THEN <signature_items> [ ELSE <signature_items> ] (END | ENDIF)
     INCLUDE <string>

  In expressions:

     IFDEF <uident> THEN <expression> [ ELSE <expression> ] (END | ENDIF)
     IFNDEF <uident> THEN <expression> [ ELSE <expression> ] (END | ENDIF)
     DEFINE <lident> = <expression> IN <expression>
     __FILE__
     __LOCATION__

  In patterns:

     IFDEF <uident> THEN <pattern> ELSE <pattern> (END | ENDIF)
     IFNDEF <uident> THEN <pattern> ELSE <pattern> (END | ENDIF)

  As Camlp4 options:

     -D<uident> or -D<uident>=expr   define <uident> with optional value <expr>
     -U<uident>                      undefine it
     -I<dir>                         add <dir> to the search path for INCLUDE'd files

  After having used a DEFINE <uident> followed by "= <expression>", you
  can use it in expressions *and* in patterns. If the expression defining
  the macro cannot be used as a pattern, there is an error message if
  it is used in a pattern.

  You can also define a local macro in an expression usigng the DEFINE ... IN form.
  Note that local macros have lowercase names and can not take parameters.

  If a macro is defined to = NOTHING, and then used as an argument to a function,
  this will be equivalent to function taking one less argument. Similarly,
  passing NOTHING as an argument to a macro is equivalent to "erasing" the
  corresponding parameter from the macro body.

  The toplevel statement INCLUDE <string> can be used to include a
  file containing macro definitions and also any other toplevel items.
  The included files are looked up in directories passed in via the -I
  option, falling back to the current directory.

  The expression __FILE__ returns the current compiled file name.
  The expression __LOCATION__ returns the current location of itself.

*)