    Display possible completions depending on the options.
    
    Intended to be used from within a shell function generating possible
    completions.  If the optional WORD argument is present, generate matches
    against WORD.
    
    If the -V option is supplied, store the possible completions in the indexed
    array VARNAME instead of printing them to the standard output.
    
    Exit Status:
    Returns success unless an invalid option is supplied or an error occurs.
