@Carl Love 

Thank you for your prompt reply.

At home I have installed only Maple 13, and it does not work. I'll give a try at work with Maple 18. Hopefully it will work there. Thanks again for your time

Dear Alejandro Jakubi

Thank you 4 ur reply.

I should have made it more general, the cos are not necessarily symmetric.

This is a fragment of my code, which goes on. the inert form might not help, i suppose.

any more tips?

@Carl Love Thank you for your time and clearness, Carl Love.

Thank you guys for your time, question answered!