Goal:
  
  G : t
  HI : invariants
  s : VSet.t
  x : V.t
  p : SPath s x x
  HG : weight (to_pathOf p) <= 0
  H : (Some 0 <= lsp0 s x x)%nbar
  H0 : 0 <= weight (to_pathOf p)
  ============================
   (Some (weight (to_pathOf p)) = Some 0)


DEBUG:Going to execute:
f_equal