  The negative binomial distribution is self similar: If the spectrum over the
whole rapidity range gives rise to a negative binomial, in absence of
correlation and if the source is unique, also a partial range in rapidity gives
rise to the same distribution. The property is not seen in experimental data,
which are rather consistent with the presence of a number of independent
sources. When multiplicities are very large self similarity might be used to
isolate individual sources is a complex production process.
