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:
unfold VSet.Empty, VSet.Subset, VSet.Equal in *
Level 1: Backtrace:

s is used in conclusion.
Level 1: Backtrace:

s is used in conclusion.
Level 1: Backtrace:

s is used in conclusion.
Level 0: Backtrace:

s is used in conclusion.