The help_filter field in a struct argp is a pointer to a
function to filter the text of help messages before displaying them.
They have a function signature like:
char *help-filter (int key, const char *text, void *input)
where key is either a key from an option, in which case text is that option's help text (see section Specifying Options in an Argp Parser), or one of the special keys with names beginning with `ARGP_KEY_HELP_', describing which other help text text is (see section Special Keys for Argp Help Filter Functions).
The function should return either text, if it should be used
as-is, a replacement string, which should be allocated using
malloc, and will be freed by argp, or zero, meaning `print
nothing'. The value of text supplied 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 zero, if
argp_help was called directly by the user.
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