  A validity of a recent estimate of an upper limit of charm production in
central Pb+Pb collisions at 158 AGeV is critically discussed. Within a simple
model we study properties of the background subtraction procedure used for an
extraction of the charm signal from the analysis of dilepton spectra. We
demonstrate that a production asymmetry between positively and negatively
charged background muons and a large multiplicity of signal pairs leads to
biased results. Therefore the applicability of this procedure for the analysis
of nucleus-nucleus data should be reconsidered before final conclusions on the
upper limit estimate of charm production could be drawn.
