Goal:
  
  s : VSet.t
  s' : VSet.t
  s'' : VSet.t
  da : forall a : VSet.elt, VSet.In a s -> VSet.In a s'
  i : VSet.elt
  disj : ~ VSet.In i (VSet.inter s' s'')
  ininter : VSet.In i (VSet.inter s s'')
  ============================
   (VSet.In i (VSet.inter s' s''))


DEBUG:Going to execute:
match goal with
           | H:?T
             |- _ =>
                 progress change T with V.t in H;
                  repeat
                   match goal with
                   | J:_ |- _ => progress change T with V.t in J
                   | |- _ => progress change T with V.t
                   end
           | H:forall x : ?T, _
             |- _ =>
                 progress change T with V.t in H;
                  repeat
                   match goal with
                   | J:_ |- _ => progress change T with V.t in J
                   | |- _ => progress change T with V.t
                   end
           end