[(content_id, FeatureCollection)] -> [Label] -> [({-1,1}, i, j)]

    where 0 <= i, j < len(content_objs).

def labels_to_indexed_coref_values(content_objs, labels):
    '''[(content_id, FeatureCollection)] -> [Label] -> [({-1,1}, i, j)]

    where 0 <= i, j < len(content_objs).
    '''
    cids_to_idx = {}
    for i, (content_id, _) in enumerate(content_objs):
        cids_to_idx[content_id] = i
    idx = lambda cid: cids_to_idx[cid]
    labs = []
    for lab in labels:
        if lab.content_id1 in cids_to_idx and lab.content_id2 in cids_to_idx:
            labs.append((lab.value.value, idx(lab.content_id1), idx(lab.content_id2)))
    return labs