Goal:
    x : VSet.elt  p : VSet.t  H : ~ VSet.In x p
  ============================
   (VSet.Equal (VSet.remove x (VSet.add x p)) p)


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

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