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:
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)
DEBUG LTAC Evaluated term: (first_in p)
DEBUG LTAC Evaluated term: (first_in p)