Goal:
  
  x : V.t
  s0 : VSet.t
  s1 : VSet.t
  s2 : VSet.t
  disj20 : Disjoint s2 s0
  disj0' : Disjoint s1 s0
  _a_ : VSet.elt
  eq : VSet.E.eq _a_ x
  ins0 : VSet.In x s1
  _a1_ : VSetProp.Add x s1 s2
  __view_subject__ : ~ VSet.In x s1
  ============================
   (False -> False)


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:~ _ |- _ => solve [ case H; trivial ]
              end ])