Goal:
    x : VSet.elt  p : VSet.t
  ============================
   (VSet.Equal (VSet.add x (VSet.add x p)) (VSet.add x p))


DEBUG:Going to execute:
match type of P with
           | Prop =>
               match goal with
               | H':context [ H ] |- _ => clear dependent H'
               end
           | _ => fail
           end