| (C.Cast (te,ty), t2) -> fo_unif_subst subst context metasenv te t2
| (t1, C.Cast (te,ty)) -> fo_unif_subst subst context metasenv t1 te
| (C.Prod (n1,s1,t1), C.Prod (_,s2,t2)) ->
| (C.Cast (te,ty), t2) -> fo_unif_subst subst context metasenv te t2
| (t1, C.Cast (te,ty)) -> fo_unif_subst subst context metasenv t1 te
| (C.Prod (n1,s1,t1), C.Prod (_,s2,t2)) ->