g(r) = g(r0), r ∈ rs, r0 ∈ rs, ∀ r0. r0 ∈ rs ∧ r0 ≠ r → g(r0) ≠ g(r) ⊦ # rs = 0, g(r) = a0, r = null, r = r0, rs = ∅