Definition 5.72.
For acyclic classes \(K\) and \(L\), define their product as the acyclic class
\[KL \quad := \quad (K \ssquare L)^{m},\]
where \(K \ssquare L\) is the class of morphisms of the form \(u \ssquare v\) for \(u \in K\) and \(v \in L\).