Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  s0 : VSet.t
  x : V.t
  y : V.t
  d : DisjointAdd x s0 (V G)
  e : EdgeOf G x y
  p : SPath G s0 y (s G)
  IHp : let H := CoreTactics.block in
        forall (x : V.t) (z : Z) (p0 : SPath G (V G) x (s G)),
        {|
          pr1 := {| pr1 := V G; pr2 := {| pr1 := x; pr2 := s G |} |};
          pr2 := p0
        |} =
        {|
          pr1 := {| pr1 := s0; pr2 := {| pr1 := y; pr2 := s G |} |}; pr2 := p
        |} ->
        let H0 := CoreTactics.block in
        sweight p0 = z ->
        ~ VSet.In x (V G) -> let H1 := CoreTactics.block in z = 0
  z0 : Z
  wp : sweight (spath_step G s0 (V G) x y (s G) d e p) = z0
  ============================
   (~ VSet.In x (V G) -> let H := CoreTactics.block in z0 = 0)


DEBUG:Going to execute:
intro