docs: def _build_prior(self, prior_tensors):
          """
          Build a tf expression for the prior by summing all child-parameter priors.
          """
          # TODO(@awav): What prior must represent empty list of parameters?
          if not prior_tensors:
              return tf.constant(0, dtype=settings.float_type)
          return tf.add_n(prior_tensors, name='prior')