data Elem : a -> List a -> Type where
     Here : Elem x (x :: xs)
     There : Elem x xs -> Elem x (y :: xs)

interface DecEq a where
  decEq : (x : a) -> (y : a) -> Dec (x = y)

zeroNotSucc : Z = (S k) -> Void
zeroNotSucc Refl impossible

recNotEq : (k = j -> Void) -> (S k) = (S j) -> Void

DecEq Nat where
  decEq Z Z = Yes Refl
  decEq Z (S k) = No zeroNotSucc
  decEq (S k) Z = No ?succNotZero
  decEq (S k) (S j) with (decEq k j)
    decEq (S j) (S j) | (Yes Refl) = Yes Refl
    decEq (S k) (S j) | (No contra) = No ?recNotEqLift

isElem : DecEq a => (x : a) -> (xs : List a) -> Maybe (Elem x xs)
isElem x [] = Nothing
isElem x (y :: xs) with (decEq x y)
  isElem y (y :: xs) | (Yes Refl) = Just Here
  isElem x (y :: xs) | (No contra) with (isElem x xs)
    isElem x (y :: xs) | (No contra) | Nothing = Nothing
    isElem x (y :: xs) | (No contra) | (Just z) = Just (There z)