--- self[key]

    key ˴ϢŤ줿֤ͤޤ륭
    ϿƤʤˤϡǥե(ꤵƤʤˤ
    nil) ֤ޤ
    ͤȤƤ nil ȶ̤ɬפ fetch
    ȤäƤ

