(* Operations for environments *)
let empty = []
let shift (ident : identifier) binding env = (ident,binding) :: env
-let rec lookup (sought_ident : ident) (env : env) : term option =
+let rec lookup (sought_ident : identifier) (env : env) : term option =
match env with
| [] -> None
| (ident, binding) :: _ when ident = sought_ident -> Some binding