Definition fun_base (T T' : eqType) (h : T' -> T) f f' :=
  rel_base h (frel f) (frel f').Definition arc p x y := let px := rot (index x p) p in take (index y px) px.

(* Specifications: *)
Lemma left_arc x y p1 p2 (p := x :: p1 ++ y :: p2) : uniq p -> arc p x y = x :: p1.
Lemma right_arc x y p1 p2 (p := x :: p1 ++ y :: p2) : uniq p -> arc p y x = y :: p2.