To get the probability of some w given an unordered subtree u, we want to calculate the probability of w given that G has generated the particular multiset W of dependents corresponding to u.
to do this, we calculate: where and W is the set of all possible configurations (w l , w r ) compatible with multiset W. W is the set of pairs of permutations of multisets W l and W r for all possible partitions of W into W l and W r .