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:
clear H
Level 1: Backtrace:

In environment
s, s'' : VSet.t
x : V.t
hin : ~ VSet.In x s
H : ~ VSet.In x s''
The term "VSet.t" has type "Type" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce VSet.t.u0 <= Prop).