Goal:
  
  x : V.t
  p : VSet.t
  H : VSet.In x p -> False
  a : V.t
  H0 : x = a -> False
  H1 : VSet.In a p
  ============================
   ((x = a \/ VSet.In a p) /\ x <> a)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro; clear id