symbol non-commutative?

Hi, I wonder if it is possible to make symbols non-commutable. The reason is that I want to invert systems of matrices, [[A B], [C D]] where A, B, C and D are matrices. The matrices are invertible, but I do not want to say anything about their structure. I have made a subclass mystring of basic as described in sec. 6.3. and redefined return_type: unsigned return_type(void) const { return return_types::noncommutative; } and it works fine: (a+b)*(a+b) = a*a+a*b+b*a+b*b However, when I make a matrix of and try to solve it, the program abort. I have also tried to make a subclass of symbol, but this class does not seem to be as easy to subclass as basic. Any hints ? Kent-Andre Mardal