We describe the structure of the model, built plugging the entry/exit decisions into a macroeconomic system, by using a notion of statistical distribution of expectations that is consistent with the idea of rational expectations (at least in its original formulation) to model the entry decision of potential entrants. The theoretical framework is also useful to analyze, on a theoretical ground, the behavior of the firms’ markups over the cycle and is employed for the agent-based simulations. In particular, we model a macroeconomic system with oligopoly, entry/exit, and heterogeneous individuals. The algebraic framework of a new macro-model is analytically dissected, to prepare a sound basis for the experiments in simulation.