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:
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.