 The behavior of the compound module is described by the set of traces  \[{\{ \varepsilon, \mathbf{?}_\textrm{in}, \mathbf{?}_\textrm{in}  x,  \mathbf{?}_\textrm{in}  x  \mathbf{?}_\textrm{out}', \mathbf{?}_\textrm{in}  x  \mathbf{?}_\textrm{out}'  n_\textrm{in}', \mathbf{?}_\textrm{in}  x  \mathbf{?}_\textrm{out}'  n_\textrm{in}'  y,  \mathbf{?}_\textrm{in}  x  \mathbf{?}_\textrm{out}'  n_\textrm{in}'  y  n+2_\textrm{out}, \ldots \}}\]   The symbols \(x,y\) can now be hidden, representing the behavior of the final system