matrices and expressions

Hi, I only just started using ginac and there's some things about matrices and their relation to expressions that I do not understand. As for instance: why does GiNaC::unity_matrix return a GiNaC::ex ? or why does A + B with A and B both a GiNaC::matrix return a GiNaC::ex? And how do I convert such a GiNaC::ex to a matrix? Can this be done? Greetings, Johan Rutgeerts