Goal:
  
  x : V.t
  p : VSet.t
  H : ~ VSet.In x p
  a : V.t
  H0 : VSet.In x p \/ ~ VSet.In x p
  H1 : x = a \/ x <> a
  ============================
   (((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:
repeat match goal with
                  | _:~ ?P, H:?P \/ ~ ?P |- _ => clear H
                  end
Level 3: Backtrace:

Tactic failure (level 1).
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

No matching clauses for match.