Goal:
    x : VSet.elt  y : VSet.elt  p : VSet.t
  ============================
   (VSet.Equal (VSet.add x (VSet.add y p)) (VSet.add y (VSet.add x p)))


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

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