--- self[nth]=val

    nth ܤΥХȤʸ val ֤ޤ
    val  0  255 ϰϤǤ硢ʸ
    ȤߤʤƤʸ֤ޤ

    val ֤ޤ

--- self[nth, len]=val

    nth ХܤĹ len ХȤʬʸ
    ʸ val ֤ޤnth ξ
    ʸޤ

    val ֤ޤ

--- self[substr]=val

    ʸ substr ˰פǽʬʸʸ
    val ֤ޤ

    self  substr ޤޤʤ硢㳰 IndexError 
    ȯޤ

    val ֤ޤ

--- self[regexp]=val
--- self[regexp, nth]=val       ruby 1.7 feature

    ɽ regexp ˥ޥåǽʬʸʸ 
    val ֤ޤ

    ɽޥåʤ㳰 IndexError ȯޤ

    val ֤ޤ

    ruby 1.7 feature:
     nth ꤷϡɽ regexp  
    nth ܤγ̤˥ޥåǽʬʸʸ val 
    ֤ޤnth  0 ξϡޥåʬʸ
     val ֤ޤ

    ɽޥåʤ nth б̤
    ʤ㳰 IndexError ȯޤ

--- self[first..last]=val
--- self[first...last]=val

    first  last ޤǤʬʸʸ val 
    ֤ޤ

    val ֤ޤ

