Goal:
  
  s : VSet.t
  s'' : VSet.t
  x : V.t
  hin : ~ VSet.In x s
  H : ~ VSet.In x s''
  ============================
   (~ VSet.In x (VSet.union s s''))


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

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

Cannot find any non-recursive equality over x.