diff --git a/eval.ml b/eval.ml
index ccf01c7..4e9c8d6 100644
--- a/eval.ml
+++ b/eval.ml
@@ -241,18 +241,25 @@ module External = struct
 end
 
 let find_operator op ts =
-  let filter t =
-    List.filter (fun (ts, _) ->
-        match ts with [] -> false | x::_ -> t=x)
+  let open List in
+  let filter_type t i =
+    filter (fun (ts, _) ->
+        nth_opt ts i
+        |> Option.map ((=) t)
+        |> Option.value ~default: false)
   in
-  let rec aux ops = function
-    | [] -> List.nth_opt ops 0
+  let rec aux ops i = function
+    | [] ->
+      ops
+      |> filter (fun (ts, _) -> length ts = i)
+      |> Fun.flip nth_opt 0
     | t::ts ->
-      (match aux (filter t ops) ts with
-      | None -> Option.bind (Type.supertype t) (fun t -> aux ops (t::ts))
-      | Some _ as x -> x)
+      (match aux (filter_type t i ops) (i+1) ts with
+       | None -> Option.bind (Type.supertype t)
+                   (fun t -> aux ops i (t::ts))
+       | Some _ as x -> x)
   in
-  aux (Operator.get op) ts
+  aux (Operator.get op) 0 ts
 
 let promote_values =
   let rec promote_until t v =