def simple_chained_rep_rnn_phi(t1, t2):
    """Map `t1` and `t2` to a single list of leaf nodes.
    
    A slight variant might insert a designated boundary symbol between 
    the premise leaves and the hypothesis leaves. Be sure to add it to 
    the vocab in that case, else it will be $UNK.
    """
    return t1.leaves() + t2.leaves()