Goal:
    x : V.t  s : VSet.t  y : VSet.elt
  ============================
   ((VSet.In y (VSet.add x s) -> x = y \/ VSet.In y (VSet.remove x s)) /\
    (x = y \/ VSet.In y (VSet.remove x s) -> VSet.In y (VSet.add x s)))


DEBUG:Going to execute:
clear H
Level 1: Backtrace:

In environment
x : V.t
s : VSet.t
y : VSet.elt
The term "VSet.elt" has type "Type" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce VSet.E.t.u0 <= Prop).