M ** N -- produce the tensor product of two modules.
Since M and N may be provided as submodules or subquotient modules, it may be necessary to replace them by quotient modules in the course of the computation, but the generators provided in the resulting tensor product will correspond to the tensor products of the generators, i.e., the modules
cover M ** cover N and
cover(M ** N) are equal. This makes it easier to make
M ** N into a functor.