**

x ** y -- a binary operator used for tensor product and cartesian product.

The user may install binary methods for this operator with code such as

              X ** Y := (x,y) -> ...

where X is the class of x and Y is the class of y .Examples:

Set ** Set -- cartesian product of sets

Matrix ** Matrix -- tensor product of matrices

Module ** Module -- tensor product of modules

Matrix ** Module -- tensor product, degree shifting

Module ** Ring -- tensor product

Matrix ** Ring -- tensor product

Monoid ** Monoid -- tensor product

Ring ** Ring -- tensor product