= Solution
An empty constructor and a constructor containing a label and two subtrees give a <polymorphic> <algebraic data type>:
``
data STree a = Empty | Node (STree a) a (STree a)
deriving (Eq, Show)
``
The <binary search tree> invariant is $\boxed{\text{every left label}<x<\text{every right label}}$ at each `Node left x right`, together with the same invariant recursively in both subtrees. Labels are distinct because the representation is of a <set>. The declaration itself needs no ordering constraint; operations that compare labels require `Ord a` in their <type signatures>.
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