Verifyies that `ebt` is the inverse of the `edgeBySourceId` data member of `tree`

def _check_rev_dict(tree, ebt):
    """Verifyies that `ebt` is the inverse of the `edgeBySourceId` data member of `tree`"""
    ebs = defaultdict(dict)
    for edge in ebt.values():
        source_id = edge['@source']
        edge_id = edge['@id']
        ebs[source_id][edge_id] = edge
    assert ebs == tree['edgeBySourceId']