--- readline([rs])

    IO#gets [IO/gets] Ʊͤ 1 ɤ߹ߤʸ֤ޤ
    EOF ã㳰 EOFError ȯޤ

    Ԥζڤϰ rs ǻꤷʸˤʤޤrs 
    ǥեͤϥƥѿ $/ ͤǤ

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

    readline  gets Ʊɤ߹ʸѿ $_ 
    ˥åȤޤ

