Goal:
    x : V.t  s0 : VSet.t  s1 : VSet.t  s2 : VSet.t
  ============================
   (DisjointAdd x s1 s2 -> Disjoint s2 s0 -> DisjointAdd x s0 (VSet.add x s0))


DEBUG:Going to execute:
move  =>disjadd disj20