code: def set_moments(self, sx, sxp, sxxp):
          
          self._sx   = sx
          self._sxp  = sxp
          self._sxxp = sxxp
          emit = _np.sqrt(sx**2 * sxp**2 - sxxp**2)
          self._store_emit(emit=emit)
docs: Sets the beam moments directly.
  
          Parameters
          ----------
          sx : float
              Beam moment where :math:`\\text{sx}^2 = \\langle x^2 \\rangle`.
          sxp : float
              Beam moment where :math:`\\text{sxp}^2 = \\langle x'^2 \\rangle`.
          sxxp : float
              Beam moment where :math:`\\text{sxxp} = \\langle x x' \\rangle`.