code: def _op(self, line, op=None, offset=0):
          
          if op is None:
              op = self.op_count[line]
          return "line{}_gate{}".format(line, op + offset)
docs: Returns the gate name for placing a gate on a line.
  
          :param int line: Line number.
          :param int op: Operation number or, by default, uses the current op count.
          :return: Gate name.
          :rtype: string