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


DEBUG:Going to execute:
unfold VSet.Empty, VSet.Subset, VSet.Equal in *
Level 1: Backtrace:

s' is used in conclusion.
Level 1: Backtrace:

s' is used in conclusion.
Level 1: Backtrace:

s' is used in conclusion.
Level 0: Backtrace:

s' is used in conclusion.