def precision_system_output(entity, partitioned_entity):
    """ Compute a spanning tree from antecedent information.

    All edges in the spanning tree correspond to anaphor-antecedent pairs. In
    order to access this antecedent information, the attribute "antecedent" of
    the mentions in the entity must be set.

    Args:
        entity (EntityGraph): The EntityGraph for the entity for which the
            spanning tree should be computed.
        partitioned_entity (EntityGraph): A partition of the entity -- not
            used for this algorithm.

    Returns:
        list(Mention, Mention): A list of mention pairs, which constitute the
        edges of the spanning tree. For a pair (m, n), n appears later in
        the text than m.
    """
    edges = []
    for mention in entity.edges:
        # just look at system output
        if ("antecedent" in mention.attributes
                and mention.attributes["antecedent"] in entity.edges[mention]):
            edges.append((mention, mention.attributes["antecedent"]))

    return sorted(edges)