Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  xs : V.t
  _b_ : Z
  xt : V.t
  ys : V.t
  _b1_ : Z
  _z_ : VSet.t
  x0 : V.t
  ============================
   (VSet.Equal (VSet.add x0 _z_) (VSet.add x0 _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
xt, ys : V.t
_b1_ : Z
_z_ : VSet.t
x0 : V.t
The term "Z" has type "Set" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce Set <= Prop).