  Recently, Land and Belew [Phys. Rev. Lett. 74, 5148 (1995)] have shown that
no one-dimensional two-state cellular automaton which classifies binary strings
according to their densities of 1's and 0's can be constructed. We show that a
pair of elementary rules, namely the ``traffic rule'' 184 and the ``majority
rule'' 232, performs the task perfectly. This solution employs the second order
phase transition between the freely moving phase and the jammed phase occurring
in rule 184. We present exact calculations of the order parameter in this
transition using the method of preimage counting.
