Goal:
  
  G : t
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  d : DisjointAdd x s0 s'
  e : EdgeOf x y
  p : SPath s0 y z
  IHp : forall u : VSet.elt,
        {VSet.In u (snodes p)} + {u = z} ->
        SPath (VSet.remove u s0) y u × SPath s0 u z
  u : VSet.elt
  Hu : u = z
  Hx : x <> u
  ============================
   (u = z)


DEBUG:Going to execute:
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"