--- each([rs]) {|line| ... }
--- each_line([rs]) {|line| ... }

    ʸγƹԤФƷ֤ޤԤζڤ rs ˻
    ʸǡΥǥեȤѿ $/ ͤǤ
     line ˤ϶ڤʸޤߤޤ

    rs  nil ꤹȹԶڤʤȤߤʤޤ
    ʸ "" ꤹϢ³ԤԤζڤȤߤʤޤ
    (ѥ饰ե⡼)

    self ֤ޤ

