canonical ordering

Hi, I have been extending GiNaC in various ways, but something I haven't yet fathomed is the canonical ordering that automatically happens. For example, I derived a class from indexed, whose compare_same_type simply calls the inherited::compare_same_type, yet the ordering of products in my class was different from that of the usual indexed objects. What determines and triggers the canonical ordering? Perhaps this could also be explained somewhere in the reference. sincerely, Garnet