def partition_pair(bif_point):
n = float(sum(<NUM_LIT:1> for _ in bif_point.children[<NUM_LIT:0>].ipreorder()))<EOL>m = float(sum(<NUM_LIT:1> for _ in bif_point.children[<NUM_LIT:1>].ipreorder()))<EOL>return (n, m)<EOL>
Calculate the partition pairs at a bifurcation point

    The number of nodes in each child tree is counted. The partition
    pairs is the number of bifurcations in the two daughter subtrees
    at each branch point.