unction to
   deal with parsing one, documentation string, a possible vector of child
   argp's, and perhaps a function to filter help output.  When actually
   parsing options, getopt is called with the union of all the argp
   structures chained together through their CHILD pointers, with conflicts
   being resolved in favor of the first occurrence in the chain.  */
struct argp
{
  /* An array of argp_option structures, terminated by an entry with both
     NAME and KEY having a value of 0.  */
  const struct argp_option *options;

  /* What to do with an option from this structure.  KEY is the key
     associated with the option, and ARG is any associated argument (NULL if
     none was supplied).  If KEY isn't understood, ARGP_ERR_UNKNOWN should be
     returned.  If a non-zero, non-ARGP_ERR_UNKNOWN value is returned, then
     parsing is stopped immediately, and that value is returned from
     argp_parse().  For special (non-user-supplied) values of KEY, see the
     ARGP_KEY_ definitions below.  */
  argp_parser_t parser;

  /* A string describing what other arguments are wanted by this program.  It
     is only used by argp_usage to print the `Usage:' message.  If it
     contains newlines, the strings separated by them are considered
     alternative usage patterns, and printed on separate lines (lines after
     the first are prefix by `  or: ' instead of `Usage:').  */
  const char *args_doc;

  /* If non-NULL, a string containing extra text to be printed before and
     after the options in a long help message (separated by a vertical tab
     `\v' character).  */
  const char *doc;

  /* A vector of argp_children structures, terminated by a member with a 0
     argp field, pointing to child argps should be parsed with this one.  Any
     conflicts are resolved in favor of this argp, or early argps in the
     CHILDREN list.  This field is useful if you use libraries that supply
     their own argp structure, which you want to use in conjunction with your
     own.  */
  const struct argp_child *children;

  /* If non-zero, this should be a function to filter the output of help
     messages.  KEY is either a key from an option, in which case TEXT is
     that option's help text, or a special key from the ARGP_KEY_HELP_
     defines, below, describing which other help text TEXT is.  The function
     should return either TEXT, if it should be used as-is, a replacement
     string, which should be malloced, and will be freed by argp, or NULL,
     meaning `print nothing'.  The value for TEXT is *after* any translation
     has been done, so if any of the replacement text also needs translation,
     that should be done by the filter function.  INPUT is either the input
     supplied to argp_parse, or NULL, if argp_help was called directly.  */
  char *(*help_filter) (int __key, const char *__text, void *__input);

  /* If non-zero the strings used in the argp library are translated using
     the domain described by this string.  Otherwise the currently installed
     default domain is used.  */
  const char *argp_domain;
};

/* Possible KEY arguments to a help filter function.  */
#define ARGP_KEY_HELP_PRE_DOC	0x2000001 /* Help text preceding options. */
#define ARGP_KEY_HELP_POST_DOC	0x2000002 /* Help text following options. */
#define ARGP_KEY_HELP_HEADER	0x2000003 /* Option header string. */
#define ARGP_KEY_HELP_EXTRA	0x2000004 /* After all other documentation;
					     TEXT is NULL for this key.  */
/* Explanatory note emitted when duplicate option arguments have been
   suppressed.  */
#define ARGP_KEY_HELP_DUP_ARGS_NOTE 0x2000005
#define ARGP_KEY_HELP_ARGS_DOC	0x2000006 /* Argument doc string.  */