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
  u : VSet.elt
  IHp : SPath (VSet.remove u s0) y u × SPath s0 u z
  Hu : {VSet.In u (VSet.add x (snodes p))} + {u = z}
  Hx : x <> u
  Hu' : {VSet.In u (snodes p)} + {u = z}
  ============================
   (DisjointAdd x s0 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"