Goal:
    s' : VSet.t  t0 : VSet.t  a : V.t  H : VSet.In a s'
  ============================
   (VSet.In a (VSet.union t0 s'))


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

Cannot find any non-recursive equality over t0.
Level 1: Backtrace:

Cannot find any non-recursive equality over t0.