--- OptionParser#on(*opts) [{...}]
--- OptionParser#def_option(*opts) [{...}]
--- OptionParser#on_head(*opts) [{...}]
--- OptionParser#def_head_option(*opts) [{...}]
--- OptionParser#on_tail(*opts) [{...}]
--- OptionParser#def_tail_option(*opts) [{...}]
    Defines option switch and handler. on_head, def_head_option
    and on_tail, def_tail_option put the switch at head
    and tail of summary, respectively.

    cf. OptionParser#switch.

