/**
* Provides a stream of the elements of the tree at each level, in level order.
*
* @return The elements of the tree at each level.
*/
public Stream<Stream<A>> levels() {
final F<Stream<Tree<A>>, Stream<Tree<A>>> flatSubForests =
Stream.<Tree<A>, Tree<A>>bind_()
.f(compose(P1.<Stream<Tree<A>>>__1(), Tree.<A>subForest_()));
final F<Stream<Tree<A>>, Stream<A>> roots = Stream.<Tree<A>, A>map_().f(Tree.<A>root_());
return iterateWhile(flatSubForests, Stream.<Tree<A>>isNotEmpty_(), single(this)).map(roots);
}
/**
* Folds a Tree<A> into a Tree<B> by applying the function f from the bottom of the Tree to the
* top
*
* @param t A tree to fold from the bottom to the top.
* @param f A function transforming the current node and a stream of already transformed nodes
* (its children) into a new node
* @return The folded tree
*/
public static <A, B> Tree<B> bottomUp(Tree<A> t, final F<P2<A, Stream<B>>, B> f) {
final F<Tree<A>, Tree<B>> recursiveCall = a -> bottomUp(a, f);
final Stream<Tree<B>> tbs = t.subForest()._1().map(recursiveCall);
return node(f.f(P.p(t.root(), tbs.map(Tree.getRoot()))), tbs);
}
/**
* Maps the given function over this tree.
*
* @param f The function to map over this tree.
* @return The new Tree after the function has been applied to each element in this Tree.
*/
public <B> Tree<B> fmap(final F<A, B> f) {
return node(
f.f(root()), subForest().map(Stream.<Tree<A>, Tree<B>>map_().f(Tree.<A, B>fmap_().f(f))));
}