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


DEBUG:Going to execute:
match goal with
           | H:context [ ?P -> False ] |- _ => fold (~ P) in H
           | |- context [ ?P -> False ] => fold (~ P)
           end