  Quantitative evaluation of some recent 'slow light' experiments based on
coherent population oscillations (CPO) shows that they can be more simply
interpreted as saturable absorption phenomena. Therefore they do not provide an
unambiguous demonstration of 'slow light'. Indeed a limiting condition on the
spectral bandwidth is not generally satisfied, such that the requirements for
burning a narrow spectral hole in the homogeneously broadened absorption line
are not met. Some definitive tests of 'slow light' phenomena are suggested,
derived from analysis of phase shift and pulse delay for a saturable absorber
