+#a #V1 #T1 #U2 #H elim (tpr_inv_bind1 … H) -H *
+[ #V2 #T #T2 #HV12 #HT1 #HT2
+ lapply (tps_inv_refl_SO2 … HT2 ???) -HT2 // /2 width=5/
+| #T2 #_ #_ #_ #H destruct
+]
+qed-.
+
+(* Advanced forward lemmas **************************************************)
+
+lemma tpr_fwd_abst1: ∀a,V1,T1,U2. ⓛ{a}V1.T1 ➡ U2 → ∀b,I,W.
+ ∃∃V2,T2. ⓑ{b,I}W.T1 ➡ ⓑ{b,I}W.T2 &
+ U2 = ⓛ{a}V2.T2.
+#a #V1 #T1 #U2 #H #b #I #W elim (tpr_inv_bind1 … H) -H *
+[ #V2 #T #T2 #HV12 #HT1 #HT2
+ lapply (tps_inv_refl_SO2 … HT2 ???) -HT2 // /3 width=4/
+| #T2 #_ #_ #_ #H destruct
+]
+qed-.