class Gem::OptionParser::List

returned.
is returned or yielded if a block is given. If it isn't found, nil is
+pat+. If +icase+ is true, the search is case insensitive. The result
Searches list +id+ for +opt+ and the optional patterns for completion

def complete(id, opt, icase = false, *pat, &block)
  __send__(id).complete(opt, icase, *pat, &block)
end