Goal:
    G1 : t  ext : EdgeSet.t  x : VSet.elt  _z_ : VSet.t  x0 : V.t  x1 : V.t
  ============================
   (VSet.Equal (VSet.add x0 (VSet.add x1 _z_))
      (VSet.add x1 (VSet.add x0 _z_)))


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

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
_z_ : VSet.t
x0, x1 : V.t
The term "V.t" has type "Type" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce EdgeSet.E.t.u0 <= Prop).