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Provided tests for isValid function currently don't cover case when order is correct in root, but incorrect in some subtree. As a consequence, function that checks only root and its closest children will pass all the tests.

Example of incorrect function passing the tests:

isValid :: Ord a => BSTree a -> Bool
isValid Nil = True
isValid (Node x l r) = (l == Nil || value l <= x) && (r == Nil || x <= value r)

Example of invalid tree that is rated as valid by function above:

      42
  8       60
9   1

When people use recursion with height instead of minHeight it passes the tests.