Goal:
    G : t  s0 : VSet.t  s' : VSet.t  x : V.t  y : V.t  t0 : VSet.t
  ============================
   (VSet.Subset s' (VSet.union t0 s'))


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

In environment
G : t
s0, s' : VSet.t
x, y : V.t
t0 : VSet.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).