⊦ disj(T1, T2) ↔ (∀ n, m. n < # T1 ∧ m < # T2 ∧ T1[n] ≠ empty ∧ T2[m] ≠ empty → T1[n].elem ≠ T2[m].elem)
[Concretesimprules(()), Extraterms((T2T1))]