use stdlib Elem and add note why not also stdlib OpenUnion

src/Quox/Error.idr

@@ -1,5 +1,7 @@
 module Quox.Error
 
+import Data.List.Elem
+
 import Control.Monad.Identity
 import Control.Monad.Trans
 import Control.Monad.Reader
@@ -10,20 +12,6 @@ import Control.Monad.RWS
 %default total
 
 
-||| a proof that an element is in a list. unlike the standard library one, this
-||| is represented as a single number at runtime.
-public export
-data Elem : a -> List a -> Type where
-  Here  : x `Elem` (x :: xs)
-  There : x `Elem` xs -> x `Elem` (y :: xs)
-%builtin Natural Elem
-
-export
-Uninhabited (x `Elem` []) where
-  uninhabited Here      impossible
-  uninhabited (There _) impossible
-
-
 ||| a representation of a list's length. this is used in the `Ord` instance to
 ||| transform the `Ord` constraints into `Eq`s
 public export
@@ -40,6 +28,8 @@ record OneOf types where
   constructor One
   elem    : type `Elem` types
   1 value : type
+-- this is basically the same as Data.OpenUnion, but using Elem instead of
+-- AtIndex is much easier for some stuff here e.g. `get`
 
 export
 Uninhabited (OneOf []) where uninhabited x = uninhabited x.elem