Goal:
    G1 : t  G2 : t  s : VSet.t  x : V.t  y : V.t  p : SPath G2 s x y
  ============================
   (VSet.In x (V G1) -> first_in p = x)


DEBUG:Going to execute:
fun c =>
             let elimc := get_first_elim c in
             let elimfn := match elimc with
                           | fun_elim => constr:(f)
                           end in
             let elimn := match elimc with
                          | fun_elim => constr:(n)
                          end in
             let elimt := make_refine elimn elimc in
             CoreTactics.apply_args c elimt
              ltac:(fun elimc => unshelve refine_ho elimc; cbv beta)