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functor OptionKeyFn(K : ORD_KEY) : ORD_KEY where type ord_key = K.ord_key option = struct type ord_key = K.ord_key option val compare = fn (NONE, NONE) => EQUAL | (NONE, _) => LESS | (_, NONE) => GREATER | (SOME x, SOME y) => K.compare (x, y) end