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

Never de Morganed, because the conjuncts share existentially quantified variables. {:nand, [{:order, x}, {:refund, x}]} means "there is no x with both", while the de Morganed form means "there are no orders at all, or no refunds at all", which is false whenever one x has an order and a different x has a refund.

Rete.Compiler.Negation extracts it 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.