docs: 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:]
          ))