  Rapidity distributions of both E895 proton data at AGS energies and NA49 net
proton data at SPS energies can be described reasonably well with a potential
version of the UrQMD in which mean-field potentials for both pre-formed hadrons
and confined baryons are considered, with the help of a traditional coalescence
afterburner in which one parameter set for both relative distance $R_0$ and
relative momentum $P_0$, (3.8 fm, 0.3 GeV$/$c), is used. Because of the large
cancellation between the expansion in $R_0$ and the shrinkage in $P_0$ through
the Lorentz transformation, the relativistic effect in clusters has little
effect on the rapidity distribution of free (net) protons. Using a
Woods-Saxon-like function instead of a pure logarithmic function as seen by
FOPI collaboration at SIS energies, one can fit well both the data at SIS
energies and the UrQMD calculation results at AGS and SPS energies. Further, it
is found that for central Au+Au or Pb+Pb collisions at top SIS, SPS and RHIC
energies, the proton fractions in clusters are about 33$\%$, 10$\%$, and
0.7$\%$, respectively.
