let ppterm = 
  ref (fun ~context ~subst ~metasenv t -> "Please, set a pp callback")
;;

let set_ppterm f = ppterm := f;;

let ppterm ?(context=[]) ?(subst=[]) ?(metasenv=[]) t = 
  !ppterm ~context ~subst ~metasenv t
;;

module C = NCic
module R = NReference

let trivial_pp_term ~context ~subst ~metasenv = 
  let rec aux ctx = function
   | C.Rel m ->
       (try List.nth ctx (m-1)
       with Failure _ -> " -" ^ string_of_int (m - List.length context))
   | C.Const r -> R.string_of_reference r
   | C.Prod (name,s,t) -> "(\Pi "^name^":"^aux ctx s^"."^aux (name::ctx) t^")"
   | C.Lambda (name,s,t) -> 
      "(\lambda "^name^":"^aux ctx s^"."^aux (name::ctx) t^")"
   | C.LetIn (name,ty,t,b) -> 
      "(let "^name^":"^aux ctx ty^":="^aux ctx t^" in "^aux (name::ctx) b^")"
   | C.Match (_,oty,t,pl) ->
      " match " ^ aux ctx t ^ " return " ^ aux ctx oty ^ "[" ^
      String.concat "|" (List.map (aux ctx) pl) ^ "]"
   | C.Appl l -> "("^String.concat " " (List.map (aux ctx) l) ^")"
   | C.Implicit _ -> "?"
   | C.Meta (n,_) -> "?"^string_of_int n
   | C.Sort C.Prop -> "Prop"
   | C.Sort C.CProp -> "CProp"
   | C.Sort (C.Type n) -> "Type"^string_of_int n
  in 
   aux (List.map fst context)