Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  s : VSet.t
  x : V.t
  H : (Some 0 <= lsp0 s x x)%nbar
  e : lsp0 s x x = None
  ============================
   (None = Some 0)


DEBUG:Going to execute:
rewrite e in   H