Goal:
  
  s : VSet.t
  s'' : VSet.t
  x : V.t
  hin : ~ VSet.In x s
  H : VSet.In x s \/ VSet.In x s''
  H0 : VSet.In x s \/ ~ VSet.In x s
  ============================
   (VSet.In x s'')


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.