##### 25.3.4.1 Flags for Argp Options

The following flags may be or'd together in the `flags` field of a `struct argp_option`. These flags control various aspects of how that option is parsed or displayed in help messages:

`OPTION_ARG_OPTIONAL`
  The argument associated with this option is optional.
`OPTION_HIDDEN`
  This option isn't displayed in any help messages.
`OPTION_ALIAS`
  This option is an alias for the closest previous non-alias option. This means that it will be displayed in the same help entry, and will inherit fields other than `name` and `key` from the option being aliased.
`OPTION_DOC`
  This option isn't actually an option and should be ignored by the actual option parser. It is an arbitrary section of documentation that should be displayed in much the same manner as the options. This is known as a documentation option.

  If this flag is set, then the option `name` field is displayed unmodified (e.g., no ‘`--`’ prefix is added) at the left-margin where a short option would normally be displayed, and this documentation string is left in it's usual place. For purposes of sorting, any leading whitespace and punctuation is ignored, unless the first non-whitespace character is ‘`-`’. This entry is displayed after all options, after `OPTION_DOC` entries with a leading ‘`-`’, in the same group.

`OPTION_NO_USAGE`
  This option shouldn't be included in `long' usage messages, but should still be included in other help messages. This is intended for options that are completely documented in an argp's `args_doc` field. See Argp Parsers. Including this option in the generic usage list would be redundant, and should be avoided.

  For instance, if `args_doc` is `"FOO BAR\n-x BLAH"`, and the ‘`-x`’ option's purpose is to distinguish these two cases, ‘`-x`’ should probably be marked `OPTION_NO_USAGE`.