--- untrace_var(varname[, hook])

    Хѿ varname ˴Ϣդ줿եå
    ޤhook ꤵ줿ˤϤΥեå
    ޤhook ά뤫ޤ nil 
    Ϳ varname ΥեåƲޤ
    줿եå֤ޤ

    :

      $v = nil
      trace_var(:$v) {|val| puts "$v=#{val.inspect}" }
      $v = 'str'        #=> $v="str"
      untrace_var :$v
      $v = 'str'        # ʤˤϤʤ

