Goal:
  
  G : t
  HI : invariants
  H : VSet.For_all (fun x : VSet.elt => lsp x x = Some 0) (V G)
  x : V.t
  p : PathOf x x
  Hp : weight p > 0
  hw : 0 < weight p + sweight (spath_refl (V G) x)
  ============================
   (VSet.Equal (VSet.union (nodes p) (V G)) (V G))


DEBUG:Going to execute:
etransitivity
DEBUG LTAC Evaluated term: VSetProp.union_sym