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

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

