+definition d_liftable2_bi: predicate (lenv → relation term) ≝
+ λR. ∀K,T1,T2. R K T1 T2 → ∀b,f,L. ⬇*[b, f] L ≡ K →
+ ∀U1. ⬆*[f] T1 ≡ U1 →
+ ∀U2. ⬆*[f] T2 ≡ U2 → R L U1 U2.
+
+definition d_deliftable2_bi: predicate (lenv → relation term) ≝
+ λR. ∀L,U1,U2. R L U1 U2 → ∀b,f,K. ⬇*[b, f] L ≡ K →
+ ∀T1. ⬆*[f] T1 ≡ U1 →
+ ∀T2. ⬆*[f] T2 ≡ U2 → R K T1 T2.
+