Goal:
    s' : VSet.t  t0 : VSet.t
  ============================
   (VSet.Subset s' (VSet.union t0 s'))


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

In environment
s', t0 : VSet.t
The term "VSet.t" has type "Type" while it is expected to have type 
"Prop" (universe inconsistency: Cannot enforce VSet.t.u0 <= Prop).