| C.Cast (te,ty) -> C.Cast (letin_nf te, letin_nf ty)
| C.Prod (n,s,t) -> C.Prod (n, letin_nf s, letin_nf t)
| C.Lambda (n,s,t) -> C.Lambda (n, letin_nf s, letin_nf t)
| C.Cast (te,ty) -> C.Cast (letin_nf te, letin_nf ty)
| C.Prod (n,s,t) -> C.Prod (n, letin_nf s, letin_nf t)
| C.Lambda (n,s,t) -> C.Lambda (n, letin_nf s, letin_nf t)