--- IO.foreach(path[, rs]) {|line| ... }

    path ǻꤵ줿եγƹԤФƷ֤ޤ
    (open Ʊ path Ƭ "|" ʤС
    ɤνϤɤ߼ޤ)礦ɰʲΤ褦Ư򤷤ޤ

      port = open(path)
      begin
        port.each_line {|line|
           ...
        }
      ensure
        port.close
      end

    path Υץ nil ֤ޤ
    Ԥ㳰 Errno::EXXX ȯޤ

    Ԥζڤϰ rs ǻꤷʸˤʤޤrs 
    ǥեͤϥƥѿ $/ ͤǤ

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

