Goal:
  
  s0 : VSet.t
  s1 : VSet.t
  s2 : VSet.t
  x : V.t
  y : V.t
  H1 : DisjointAdd x s0 s1
  H2 : DisjointAdd y s1 s2
  z : VSet.elt
  Hz : x = z
  ============================
   (x = 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"