The negation of a conjunction: "no match satisfies all of these at once".

Never rewritten with De Morgan's law, because the conjuncts share existentially quantified variables. {:nand, [{:order, x}, {:refund, x}]} means "there is no x with both". The De Morgan form would mean "there are no orders at all, or no refunds at all" instead — false whenever one x has an order and a different x has a refund.

Rete.Compiler.Negation extracts this into a helper production whose RHS inserts a marker fact. Nothing else in the pipeline can evaluate one. It binds nothing downstream. But its inner conditions are still classified, because the helper needs their join keys.