Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  a : V.t
  ============================
   ((VSet.In a (VSet.add x0 (VSet.add x1 _z_)) ->
     VSet.In a (VSet.add x1 (VSet.add x0 _z_))) /\
    (VSet.In a (VSet.add x1 (VSet.add x0 _z_)) ->
     VSet.In a (VSet.add x0 (VSet.add x1 _z_))))


DEBUG:Going to execute:
unfold not in *
Level 0: Backtrace:

No matching clauses for match.