{-@ assume axiom_append_cons :: x:a -> xs: L a -> ys: L a
          -> {v:Proof | append (C x xs) ys == C x (append xs ys) } @-}
axiom_append_cons :: a -> L a -> L a -> Proof
axiom_append_cons x xs ys = Proof
axiom_append_cons :: a -> L a -> L a -> Proof
axiom_append_cons x xs ys = Proof
axiom_append_cons x xs ys = Proof