 where \(\circ\) means input or, more generally, a passive involvement of the module in the corresponding action, whereas \(\bullet\) means output, or active involvement in the action. We can describe the behavior of \(S\) using traces (Hoare 1985), i.e., finite sequences of symbols from the infinite alphabet \( \alpha S = {\{ \mathbf{?}_\textrm{in},\mathbf{?}_\textrm{out},n_\textrm{in},m_\textrm{out} \}}: \)   \[\tau S = {\{ \varepsilon,\mathbf{?}_\textrm{in},\mathbf{?}_\textrm{in}  \mathbf{?}_\textrm{out}, \mathbf{?}_\textrm{in}  \mathbf{?}_\textrm{out} n_\textrm{in},\mathbf{?}_\textrm{in}  \mathbf{?}_\textrm{out} n_\textrm{in}  n+1_\textrm{out},\ldots \}}\]   If we consider another instance \(S'\) of \(S\) with alphabet \( \alpha S' = {\{ \mathbf{?}_\textrm{in}',\mathbf{?}_\textrm{out}',n_\textrm{in}',m_\textrm{out}' \}} \) we can compose \(S\) and \(S'\) by identifying (= connecting) channels \(r_\textrm{out},r_\textrm{in}'\), and \(a_\textrm{in} , a_\textrm{out}'\), and the signals passing through them, as shown:  \[\begin{aligned} \mathbf{?}_\textrm{out} , \mathbf{?}_\textrm{in}' &amp;\leadsto x\\ n+1_\textrm{in} , n+1_\textrm{out}' &amp;\leadsto y\end{aligned}\]   This represents the parallel composition of the modules, \(S \| S'\):