Zip a pair of trees cropping to the smallest degree and depth, and apply the
given function.
Returns the smallest matching tree of pairs, one taken from each tree at the
same position, and run the given function on this pair, returning a tree of
its results.
Zip a pair of trees cropping to the smallest degree and depth, and apply the given function.
Returns the smallest matching tree of pairs, one taken from each tree at the same position, and run the given function on this pair, returning a tree of its results.