proposition: Lemma expand_pure e A: is_pure e -> vars e ≦ A -> e ≡ \sum_(a \in A) var a ⋅ deriv a e .
proof: Proof. intros He HA. rewrite (expand' e HA) at 1. rewrite epsilon_pure by assumption. apply cupbx. Qed.
imports: syntax kleene boolean lang lset sums normalisation