--- OptionParser::Switch#parse(arg, val) {semi-error handler}
    Parses argument, convert and returns arg, block and
    result of conversion.
    : Arguments to @conv
      substrings matched to @pattern, $&, $1,
      $2 and so on.
    :Parameters:
      : arg
        argument string follows the switch.
      : val
        following argument.
      : block
        yields at semi-error condition, instead of raises exception.

