docs: def _expectation(p, kern, feat, none2, none3, nghp=None):
      r"""
      Compute the expectation:
      <\Sum_i Ki_{X, Z}>_p(X)
          - \Sum_i Ki_{.,.} :: Sum kernel
  
      :return: NxM
      """
      return functools.reduce(tf.add, [
          expectation(p, (k, feat), nghp=nghp) for k in kern.kernels])