--- ENV.fetch(key,[default])
--- ENV.fetch(key) {|key| ... }

    key ˴ϢŤ줿֤ͤޤ륭ϿƤ
    ʤˤϡ default ͿƤФͤ򡢥֥
    ͿƤФΥ֥åɾ֤ͤޤΤ
    Ǥʤ㳰 IndexError ȯޤ

