fn empty_subtree_hash(&self, depth: usize) -> H256 {
        // We do not cache by default since then the cached values would be preserved
        // for all implementations which is not correct for different leaves.
        compute_empty_tree_hashes(self.empty_leaf_hash())[depth]
    }
Returns the hash of an empty subtree with the given depth.
Implementations are encouraged to cache the returned values.
core/lib/mini_merkle_tree/src/lib.rs