Goal:
  
  x : V.t
  s0 : VSet.t
  s1 : VSet.t
  s2 : VSet.t
  disjadd : DisjointAdd x s1 s2
  dijs20 : Disjoint s2 s0
  _a_ : VSet.elt
  ============================
   (VSet.In _a_ s1 -> VSet.In _a_ (VSet.union s2 s0))


DEBUG:Going to execute:
move  =>/(DisjointAdd_Subset disjadd) ?; apply /VSet.union_spec 
DEBUG LTAC Evaluated term: VSet.union_spec
DEBUG LTAC Evaluated term: (iffLR
                              (Q:=VSet.In _a_ s1 \/
                                  VSet.In _a_ (VSet.add x s0))
                              (VSet.union_spec s1 (VSet.add x s0) _a_)
                              _view_subject_)