Goal:
  
  G : t
  HI : invariants
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  d : DisjointAdd x s0 s'
  x0 : Z
  i : EdgeSet.In (x, x0, y) (E G)
  p : SPath s0 y z
  H : VSet.In x (V G)
  IHp : VSet.In y (V G) -> VSet.In z (V G)
  ============================
   (VSet.In (x, x0, y)..s (V G) /\ VSet.In (x, x0, y)..t (V G) ->
    VSet.In y (V G))


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"