| C.Prod (_,s,t) -> (occur uri s) or (occur uri t)
| C.Cast (te,ty) -> (occur uri te)
| C.Lambda (_,s,t) -> (occur uri s) or (occur uri t) (* or false ?? *)
| C.Prod (_,s,t) -> (occur uri s) or (occur uri t)
| C.Cast (te,ty) -> (occur uri te)
| C.Lambda (_,s,t) -> (occur uri s) or (occur uri t) (* or false ?? *)