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


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