Goal:
  
  G : t
  s : VSet.t
  y1 : V.t
  p2 : SPath s y1 y1
  HG : weight (to_pathOf p2) <= 0
  x : V.t
  y2 : V.t
  n : Z
  He : EdgeSet.In (y1, n, y2) (E G)
  Hy : VSet.In y1 s
  p : SPath s x y1
  p1 : SPath (VSet.remove y1 s) x y1
  Hp12 : split' p = (p1, p2)
  H : sweight p1 + sweight p2 = sweight p
  HH : sweight p2 = weight (to_pathOf p2)
  ============================
   (sweight p + n <= sweight p1 + n)


DEBUG:Going to execute:
zify_elim_let