Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  xs : V.t
  _b_ : Z
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  ============================
   (VSet.Equal (VSet.add x0 (VSet.add x1 (VSet.add x2 _z_)))
      (VSet.add x2 (VSet.add x0 (VSet.add x1 _z_))))


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

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
xs : V.t
_b_ : Z
_z_ : VSet.t
x0, x1, x2 : V.t
The term "Z" has type "Set" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce Set <= Prop).