Goal:
  
  G : t
  HG : acyclic_no_loop
  s : VSet.t
  x : V.t
  y1 : V.t
  y2 : V.t
  n : Z
  He : EdgeSet.In (y1, n, y2) (E G)
  Hy : VSet.In y1 s
  m : Z
  p : SPath s x y1
  p1 : SPath (VSet.remove y1 s) x y1
  p2 : SPath s y1 y1
  Hp12 : split' p = (p1, p2)
  H : sweight p1 + sweight p2 = sweight p
  ============================
   (VSet.In y1 s)


DEBUG:Going to execute:
let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"