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


DEBUG:Going to execute:
match goal with
           | H':context [ H ] |- _ => clear dependent H'
           end
DEBUG LTAC Evaluated term: (~ VSet.In x s)
DEBUG LTAC Evaluated term: Prop
Level 3: type of P
 has value 
Prop