Goal:
  
  G : t
  x : V.t
  i : V.t
  H : VSet.In i (VSet.add x VSet.empty)
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In i VSet.empty \/ i = x)


DEBUG:Going to execute:
progress change T with V.t
DEBUG LTAC Evaluated term: VSet.elt
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.