Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  x : VSet.elt
  inx : VSet.In x (V G)
  m : Z
  Hm : lsp (s G) x = Some m
  Hpos : 0 <= m
  ============================
   (exists n : Z,
      lsp (s G) x = Some n /\ 0 <= n /\ Z.of_nat (to_label (lsp (s G) x)) = n)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: Z_of_pos_alt
DEBUG LTAC Evaluated term: (lsp_s x inx)