 whose traces have the required form   \[{\{  \varepsilon, \mathbf{?}_\textrm{in}, \mathbf{?}_\textrm{in}  \mathbf{?}_\textrm{out}', \mathbf{?}_\textrm{in}   \mathbf{?}_\textrm{out}' n_\textrm{in}', \mathbf{?}_\textrm{in}   \mathbf{?}_\textrm{out}'  n_\textrm{in}'   n+2_\textrm{out}, \ldots \}}.\]   This example contains many of the ingredients on which game semantics is based. There is the idea of a System, whose behavior is triggered by an incoming request from its Environment: in a game formalism these are the roles of Proponent and Opponent in a two-person game. The behavior of each module is described as the trace of its possible interactions with other agents, and the behaviors can be composed by a peculiar change of role whereby the module who plays as System (in the above example, \(S\) emitting a request signal on channel \(r_\textrm{out}\)) is made to behave as Environment with respect to \(S'\) when this signal is received in input on channel \(r_\textrm{in}'\). Let us see how this example can be generalized.