code: def angular_momentum(self):
          r
          cart = self.represent_as(coord.CartesianRepresentation)
          return cart.pos.cross(cart.vel).xyz
docs: r"""
          Compute the angular momentum for the phase-space positions contained
          in this object::
  
          .. math::
  
              \boldsymbol{{L}} = \boldsymbol{{q}} \times \boldsymbol{{p}}
  
          See :ref:`shape-conventions` for more information about the shapes of
          input and output objects.
  
          Returns
          -------
          L : :class:`~astropy.units.Quantity`
              Array of angular momentum vectors.
  
          Examples
          --------
  
              >>> import numpy as np
              >>> import astropy.units as u
              >>> pos = np.array([1., 0, 0]) * u.au
              >>> vel = np.array([0, 2*np.pi, 0]) * u.au/u.yr
              >>> w = PhaseSpacePosition(pos, vel)
              >>> w.angular_momentum() # doctest: +FLOAT_CMP
              <Quantity [0.        ,0.        ,6.28318531] AU2 / yr>