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

def _phase(self, line, pos):
        """
        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
        """
        phase_str = "\n\\node[phase] ({}) at ({},-{}) {{}};"
        return phase_str.format(self._op(line), pos, line)