Goal:
  
  G : t
  s : VSet.t
  x : VSet.elt
  y : V.t
  k : Z
  Hx : VSet.In x s
  Hk : EdgeSet.In (x, k, y) (E G)
  ============================
   (VSet.In x s)


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"