code: def _compute_k(self, tau):
          r
          y = self._compute_y(tau)
          return y**(-self.params[1])
docs: r"""Evaluate the kernel directly at the given values of `tau`.
          
          Parameters
          ----------
          tau : :py:class:`Matrix`, (`M`, `D`)
              `M` inputs with dimension `D`.
          
          Returns
          -------
              k : :py:class:`Array`, (`M`,)
                  :math:`k(\tau)` (less the :math:`\sigma^2` prefactor).