I am implementing the dynamic programming algorithm for computing tree alignment distance as described in "Alignment of trees — an alternative to tree edit". But I got stuck in lemma 3 on page 141 (page 5 of pdf). The notations are given at the beginning of section 2. The last case in lemma 3 says
However, there is no recurrence formula given to compute . How did we obtain this?
I don't think the following holds either: ?