def add_body(self, body):
        """
        Add a :class:`Body` to the system. This function also sets the
        ``system`` attribute of the body.

        :param body:
            The :class:`Body` to add.

        """
        body.system = self
        self.bodies.append(body)
        self.unfrozen = np.concatenate((
            self.unfrozen[:-2], np.zeros(7, dtype=bool), self.unfrozen[-2:]
        ))
Add a :class:`Body` to the system. This function also sets the
        ``system`` attribute of the body.

        :param body:
            The :class:`Body` to add.