Write a predicate bst-balanced? that determines if a given Bst is balanced. A tree is balanced if each subtree is balanced and the height of the two subtrees differs by at most one. The height of a Bst is the length of the longest (maximum length) path from root to a leaf, including the root and the leaf. For example, the height of the tree on slide 08-35 in the notes is three. The height of a tree with a single node is one. An empty Bst has a height of zero and is balanced.

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