// If n == 0, returns nil, x.
func binarySearchTree(x *stackObjectBuf, idx int, n int) (root *stackObject, restBuf *stackObjectBuf, restIdx int) {
	if n == 0 {
		return nil, x, idx
	}
	var left, right *stackObject
	left, x, idx = binarySearchTree(x, idx, n/2)
	root = &x.obj[idx]
	idx++
	if idx == len(x.obj) {
		x = x.next
		idx = 0
	}
	right, x, idx = binarySearchTree(x, idx, n-n/2-1)
	root.left = left