As before, letting q = Suff k−1 (w), it follows that Suff k−1 (w) = Suff k−1 (q) so tails f (w) = tails f (q), and Suff k−1 (wa) = Suff k−1 (qa) so tails f (wa) = tails f (qa).
q, a, x, Suff k−1 (qa) ∈ δ. c .