#### 7.2.3.3 module operations (plural)

`+`
  addition (concatenation of the generators and simplification) Note that "-" implicitly converts a module into a matrix; see below example.

`*`
  right or left multiplication with number, ideal, or poly (but not `module` * `module`!)

module_expression `[` int_expression `,` int_expression `]`
  is a module entry, where the first index indicates the row and the second the column

module_expressions `[` int_expression `]`
  is a vector, where the index indicates the column (generator)

Example:

    ring A=0,(x,y,z),Dp;
    matrix D[3][3];
    D[1,2]=-z;  D[1,3]=y;  D[2,3]=x;  // this algebra is U(so_3)
    def B=nc_algebra(1,D);
    setring B;
    module M = [x,y],[0,0,x*z];
    module N = matrix((x+y-z)*M) - matrix(M*(x+y-z)); // no - for type module
    print(N);
    ==> -y-z,0,
    ==> -x+z,0,
    ==> 0,   -x2-xy-yz-z2

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