--- self[nth]

    nth ܤǤ֤ޤƬǤ 0 ܤˤʤޤ
    nth ͤλˤΥǥåȸ路ޤ(
    Ǥ -1 )nth ܤǤ¸ߤʤˤ 
    nil ֤ޤ

--- self[start..end]

    start ܤǤ end ܤǤޤǤʬ
    ޤstart ͤλˤΥǥåȸ路
    ޤ(Ǥ -1 )start ͤϰϤ˼ޤ
     nil ֤ޤend ͤĹۤ
    ˤϡۤʬĹ̵뤵ޤޤϰϤλ
    ˤ nil ֤ޤ

--- self[start, length]

    start ܤ length ĤǤޤʬ֤ޤ
    start ͤλˤΥǥåȸ路ޤ(
    Ǥ -1 )length  start ܤ
    ĹĹˤϡۤʬĹ̵뤵ޤlength 
    λˤ nil ֤ޤ

