Goal:
  
  G : t
  HG : acyclic_no_loop G
  z : V.t
  x : V.t
  e : EdgeOf G x z
  i : VSet.elt
  H : VSet.In i (VSet.add x VSet.empty)
  edges_vertices0 : forall e : EdgeSet.elt,
                    EdgeSet.In e (E G) ->
                    VSet.In e..s (V G) /\ VSet.In e..t (V G)
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (VSet.In i VSet.empty \/ i = x)


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

Proof is not complete.
Level 1: Backtrace:

Proof is not complete.
Level 1: Backtrace:

Proof is not complete.
Level 2: Backtrace:

Proof is not complete.
Level 1: Backtrace:

Proof is not complete.
Level 1: Backtrace:

Proof is not complete.
Level 1: Backtrace:

Proof is not complete.