Goal:
  
  G : t
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  p : SPath s0 x y
  t0 : VSet.t
  t1 : V.t
  hin : Disjoint t0 s'
  q : SPath s' t1 z
  ============================
   (VSet.Subset s' (VSet.union t0 s'))


DEBUG:Going to execute:
match goal with
           | H':?Y
             |- _ =>
                 match H with
                 | H' => fail 2
                 | _ => unify X Y; clear H' || clear H
                 end
           end
Level 4: Backtrace:

Tactic failure (level 2).
Level 4: Backtrace:

Tactic failure (level 1).
Level 3: Backtrace:

Tactic failure (level 1).
Level 2: Backtrace:

Tactic failure.