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Parametric Polymorphic Operations

ML hd operator.

Exprval = (Nat + Exprval^* + (Exprval × Exprval) +
    (Exprval + Exprval) + (Exprval -> Exprval) +
    Errvalue)_{_|_}

E[[hd E]] = lambda e.
    let x = E[[E]]e in
        cases x of
            isNat(n) -> inErrvalue()
            [] isExprval^*(l) -> hd l
            [] ...
        end