Goal:
  
  x : V.t
  y : V.t
  p : VSet.t
  H : x <> y
  a : V.t
  H0 : x = y \/ x <> y
  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:
assert new (x = y \/ x <> y) by
apply VSetDecide.MSetDecideAuxiliary.dec_eq