Goal:
    x : V.t  p : VSet.t  H : ~ VSet.In x p  a : V.t
  ============================
   (VSet.In a (VSet.remove x (VSet.add x p)) <-> VSet.In a p)


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

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

Cannot find any non-recursive equality over a.