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


DEBUG:Going to execute:
push not in * using MSet_decidability