def pos(self, p_x=None, y=None, z=None):
        """Set/Get actor position."""
        if p_x is None:
            return np.array(self.GetPosition())
        if z is None:  # assume p_x is of the form (x,y,z)
            self.SetPosition(p_x)
        else:
            self.SetPosition(p_x, y, z)
        if self.trail:
            self.updateTrail()
        return self