(* Basic properties *********************************************************)
-(* Basic_1: was by definition: csubst1_refl *)
lemma lpqs_refl: ∀L. L ⊢ ➤* L.
/2 width=1 by lpx_sn_refl/ qed.
lemma lpqs_fwd_length: ∀L1,L2. L1 ⊢ ➤* L2 → |L1| = |L2|.
/2 width=2 by lpx_sn_fwd_length/ qed-.
+
+(* Advanced forward lemmas **************************************************)
+
+lemma lpqs_fwd_append1: ∀K1,L1,L. K1 @@ L1 ⊢ ➤* L →
+ ∃∃K2,L2. K1 ⊢ ➤* K2 & L = K2 @@ L2.
+/2 width=2 by lpx_sn_fwd_append1/ qed-.
+
+lemma lpqs_fwd_append2: ∀L,K2,L2. L ⊢ ➤* K2 @@ L2 →
+ ∃∃K1,L1. K1 ⊢ ➤* K2 & L = K1 @@ L1.
+/2 width=2 by lpx_sn_fwd_append2/ qed-.