Goal:
  
  s : VSet.t
  x : V.t
  Hx : Z.of_nat (VSet.cardinal (VSet.remove x s)) + 1 =
       Z.of_nat (VSet.cardinal s)
  cstr : 0 <= Z.of_nat (VSet.cardinal s)
  cstr0 : 0 <= Z.of_nat (VSet.cardinal (VSet.remove x s))
  ============================
   (Z.of_nat (VSet.cardinal (VSet.remove x s)) < Z.of_nat (VSet.cardinal s))


DEBUG:Going to execute:
idtac