GeV [9] with different bias on the overall normalization of the distributions: N$_{prot}$$_{.}$ ≤ 208 (i.e., half of the net- nucleons can be participant protons) and N$_{prot}$$_{.}$ ≤ 128 (which is the hydrodynamic normalization of the dis- tribution). The error bars present largest uncertainties among mentioned above. The lower point at s 1 / 2 = 17 . 3 GeV corresponds to the new data at 158 A GeV. Its up- per error, as well as that of 80 A -GeV point, results from the uncertainty of the normalization. The irregularity observed in Fig. 1 is distinctly seen here as a “zig-zag” irregularity in the energy dependence of C$_{y}$ . It is somewhat suspicious that the “zig-zag” irregu- larity happens at the border between the AGS and SPS energies. It could imply that this irregularity results from different ways of selecting central events in AGS and SPS experiments. However, there are indirect evidences of a physical (rather than methodical) nature of this irregu- larity. The difference between C$_{y}$ values in two different experiments at 10 A GeV can be taken as an estimate of the methodical uncertainty. The difference between C$_{y}$ values at 10 A GeV and 20 A GeV is two to three times larger than this methodical uncertainty. Moreover, we could expect that C$_{y}$ at 20 A GeV would be larger than that at 10 A GeV because the incident energy is higher and centrality selection at 20 A GeV is less restrictive (7%) than at 10 A GeV (5%). Contrary to these expec- tations the C$_{y}$ at 20 A GeV is smaller than that at 10 A GeV. There should be a physical reason for that. Ex- citation functions of other quantities [21] deduced from the same AGS and SPS data do not reveal any misfit at the border between the AGS and SPS domains. The latter suggests that the AGS and SPS data were taken at similar physical conditions. However, new data taken at the same acceptance and the same centrality selection in this energy range are highly desirable to clarify this problem. Hopefully such data will come from new ac- celerators FAIR at GSI and NICA at Dubna, as well as from the low-energy-scan program at RHIC. Figure 2 also contains C$_{y}$ deduced from results of 3FD simulations with a hadronic equation of state (hadr. EoS) [19] and a EoS involving a first-order phase transition into the quark-gluon phase (2-ph. EoS) [20]. To obtain y$_{s}$ and w$_{s}$ , the 3FD spectra were also fitted by the form (1). For central (5%) Au+Au collisions at AGS energies we performed our calculations taking a fixed impact pa- rameter b = 2 fm; for the central (5%) Pb+Pb reaction at E$_{lab}$ = 158 A GeV, b = 2.4 fm which is the experimen- tal estimate for this centrality [22]; for other central (7%) Pb+Pb collisions at 20 A -80 A GeV, b = 3 fm. The 3FD model with the hadronic EoS reasonably re- produces a great body of experimental data in a wide energy range from AGS to SPS, see Ref. [17, 23–25]. De- scription of the rapidity distributions with the hadronic EoS is reported in Refs. [17, 18]. The reproduction of the The 3FD simulations have been also done with a EoS involving a first-order phase transition into the quark- gluon phase (2-ph. EoS) [20]. In 2-ph. EoS the Gibbs construction was used for the mixed phase. These calcu- lations well reproduce the AGS data up to the energy of distributions is quite good at the AGS energies and at the top SPS energies. At 40 A GeV the description is still sat- isfactory. However, at 20 A and 30 A GeV the hadr.-EoS predictions completely disagree with the data, cf. [18]. At 20 A GeV instead of a bump at the midrapidity the hadronic scenario predicts a quite pronounced dip. The problems with the description of the low-energy SPS data are clearly seen from Fig. 2 and also from Fig. 3, where midrapidity values of the rapidity distributions (multi- plied by the center-of-mass rapidity) are presented. In Ref. [18] it was demonstrated that the problem with the low-energy SPS data can be solved by considerable soft- ening the hadronic EoS. This softening may indicate an onset of the phase transition into the quark-gluon phase. Notice that a maximum in y$_{cm}$ ( dN/dy )$_{cm}$ at s 1 / 2 = 4 . 7 GeV happens only because the light fragment production becomes negligible above this energy. The 3FD calcula- tion without coalescence (i.e. without the fragment pro- duction) reveals a monotonous decrease of y$_{cm}$ ( dN/dy )$_{cm}$ beginning from s 1 / 2 = 2 . 7 GeV, i.e. from the lowest en- ergy considered here. FIG. 3: Midrapidity value of the net-proton rapidity spec- trum as a function of the center-of-mass energy of colliding nuclei. Experimental data are confronted to predictions of the 3FD model with the hadronic EoS (hadr. EoS) [19] and the EoS with a first-order phase transition (2-ph. EoS) [20]. The thin long-dashed line corresponds to the hadr.-EoS cal- culation without fragment production. III. THREE-FLUID MODEL SIMULATIONS 3 4 8 12 16 20 s$^{1/2}$ [GeV] 70 80 90 100 110 120 y cm  ( d N/dy) cm NA49 E895, E917, E866 hadr. EoS hadr. EoS w/o coal. 2-ph. EoS