Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  x : V.t
  z : Z
  p : SPath G (V G) x (s G)
  p0 := {|
          pr1 := {| pr1 := V G; pr2 := {| pr1 := x; pr2 := s G |} |};
          pr2 := p
        |}
     : sigma
         (fun index : sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t)) =>
          SPath G (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  ============================
   (let H := CoreTactics.block in
    sweight p = z ->
    ~ VSet.In x (V G) -> let H0 := CoreTactics.block in z = 0)


DEBUG:Going to execute:
generalize (eq_refl : v = id'); clearbody id'; simpl in id'