on-specific completion functions may use this
     function to present the same interface as 'rl_complete()'.

 -- Function: char ** rl_completion_matches (const char *text,
          rl_compentry_func_t *entry_func)
     Returns an array of strings which is a list of completions for
     TEXT.  If there are no completions, returns 'NULL'.  The first
     entry in the returned array is the substitution for TEXT.  The
     remaining entries are the possible completions.  The array is
     terminated with a 'NULL' pointer.

     ENTRY_FUNC is a function of two args, and returns a 'char *'.  The
     first argument is TEXT.  The second is a state argument; it is zero
     on the first call, and non-zero on subsequent calls.  ENTRY_FUNC
     returns a 'NULL' pointer to the caller when there are no more
     matches.

 -- Function: char * rl_filename_completion_function (const char *text,
          int state)
     A generator function for filename completion in the general case.
     TEXT is a partial filename.  The Bash source is a useful reference
     for writing application-specific completion functions (the Bash
     completion functions call this and other Readline functions).

 -- Function: char * rl_username_completion_function (const char *text,
          int state)
     A completion generator for usernames.  TEXT contains a partial
     username preceded by a random character (usually '~').  As with all
     completion generators, STATE is zero on the first call and non-zero
     for subsequent calls.