def _get_mti_definition(key)
      num_hash,name_hash = self.class._mti_definitions
      if num_hash[key]
        [key, num_hash[key]]
      elsif name_hash[key]
        [name_hash[key], key]
      else
        raise ISO8583Exception.new("MTI: #{key} not allowed!")
      end
    end
def _get_mti_definition(key)
      num_hash,name_hash = self.class._mti_definitions
      if num_hash[key]
        [key, num_hash[key]]
      elsif name_hash[key]
        [name_hash[key], key]
      else
        raise ISO8583Exception.new("MTI: #{key} not allowed!")
      end
    end
return [mti_num, mti_value] for key being either
 mti_num or mti_value