Goal:
    x : V.t  s : VSet.t  y : V.t
  ============================
   ((VSet.In y (VSet.add x s) -> x = y \/ VSet.In y (VSet.remove x s)) /\
    (x = y \/ VSet.In y (VSet.remove x s) -> VSet.In y (VSet.add x s)))


DEBUG:Going to execute:
subst x
Level 1: Backtrace:

Cannot find any non-recursive equality over y.
Level 1: Backtrace:

Cannot find any non-recursive equality over y.