// with expected.
func (t *T) wellFormed() (s string, i int) {
	if t.root == nil {
		s = ""
		i = 0
		return
	}
	return t.root.wellFormedSubtree(nil, -0x80000000, 0x7fffffff)
}

// wellFormedSubtree ensures that a red-black subtree meets
// all of its invariants and returns a string identifying
// the first problem encountered. If there is no problem