why is GiNaC::symbol commutative??

Hello, subject explains it all. I would like to use symbols to represent matrices. Obvious solution (replacing symbols via some_ex.sub(some_symbol==some_matrix) does not work for multiplication of matrices. Other ways to do it? Thanks. Vaclav. (If I understand it right, upon replacement of one matrix, one has matrix*symbol; symbol is supposed to be a scalar, so we have commutative multiplication; then the other symbol is also substituted for a matrix, so one gets exception "non-commutative scalar".)