(* Basic_1: was: drop1_gen_pcons *)
lemma ldrops_inv_cons: ∀L1,L2,d,e,des. ⇩*[{d, e} @ des] L1 ≡ L2 →
∃∃L. ⇩*[des] L1 ≡ L & ⇩[d, e] L ≡ L2.
(* Basic_1: was: drop1_gen_pcons *)
lemma ldrops_inv_cons: ∀L1,L2,d,e,des. ⇩*[{d, e} @ des] L1 ≡ L2 →
∃∃L. ⇩*[des] L1 ≡ L & ⇩[d, e] L ≡ L2.