Goal:
    x : V.t  y : V.t  p : VSet.t  a : V.t  H : x = a \/ y = a \/ VSet.In a p
  ============================
   (y = a \/ x = a \/ VSet.In a p)


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