  We consider a phase-space model for particle production in nuclear
collisions. Once the multiplicities of the individual particle species are
known, single-inclusive momentum spectra can be computed after making
simplifying assumptions for the matrix element for multiparticle production.
Comparison of the calculated spectra with data for pions and kaons from central
Pb+Pb collisions at E_{Lab}=158 AGeV reveals a residual longitudinal
phase-space dominance in the final state of the reaction. We account for this
by modifying the isotropic, relativistic invariant phase space in a way which
retains boost invariance in beam direction but suppresses large transverse
momenta. Adjusting a single parameter, we obtain a reasonably good description
of transverse momentum and rapidity spectra for both pions and kaons.
