need an operation `make_node` that takes two subtrees (perhaps leaves, perhaps
other nodes) and joins them into a new tree. You'll need an operation `isleaf`
that tells you whether a given tree is a leaf. And an operation `extract_label`
need an operation `make_node` that takes two subtrees (perhaps leaves, perhaps
other nodes) and joins them into a new tree. You'll need an operation `isleaf`
that tells you whether a given tree is a leaf. And an operation `extract_label`