code: def _phase(self, line, pos):
          
          phase_str = "\n\\node[phase] ({}) at ({},-{}) {{}};"
          return phase_str.format(self._op(line), pos, line)
docs: Places a phase / control circle on a qubit line at a given position.
  
          :param int line: Qubit line at which to place the circle.
          :param float pos: Position at which to place the circle.
          :return: Latex string representing a control circle at the given position.
          :rtype: string