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


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2