def pre_attention(self, segment, query_antecedent, memory_antecedent, bias):
    """Called prior to self-attention, to incorporate memory items.

    Args:
      segment: an integer Tensor with shape [batch]
      query_antecedent: a Tensor with shape [batch, length_q, channels]
      memory_antecedent: must be None. Attention normally allows this to be a
        Tensor with shape [batch, length_m, channels], but we currently only
        support memory for decoder-side self-attention.
      bias: bias Tensor (see attention_bias())
    Returns:
      (data, new_query_antecedent, new_memory_antecedent, new_bias)
    """
    del segment
    return None, query_antecedent, memory_antecedent, bias