--- OptionParser.accept(t, [pat]) {...}
--- OptionParser#accept(t, [pat]) {...}
    Directs to accept specified class argument.
    :Parameters:
      : t
        argument class specifier, any object including Class.
      : pat
        pattern for argument, defaulted to t if it respond to match.
      : Block
        receives argument string and should be convert to desired class.

