/**
* 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);
}
/**
* Builds a tree from a seed value.
*
* @param f A function with which to build the tree.
* @return A function which, given a seed value, yields a tree.
*/
public static <A, B> F<B, Tree<A>> unfoldTree(final F<B, P2<A, P1<Stream<B>>>> f) {
return b -> {
final P2<A, P1<Stream<B>>> p = f.f(b);
return node(p._1(), p._2().map(Stream.<B, Tree<A>>map_().f(unfoldTree(f))));
};
}
/**
* 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))));
}
