How can I do in do files to match the length of differential pairs?

Dear ladies and Gentlemen:

I use Specctra Autorouter 15.0. It's very nice. Thank you for your creative work. But I have some questions in using it.

It's easy to match the length of a bus. For example, I can use this command in do file: define (class address A0 A1 A2 A3 A4 A5 A6 A7), circuit class address(match_net_length on (tollerance 5)). But for some differential pairs, How can I match the length of them? For example,I use command: define (pair (nets B0_P B0_N)), define (pair (nets B1_P B1_N)),...,define (pair (nets B7_P B7_N)).
How can I do next to make all the differential pairs match length?

Thank you very much. We will become stronger because of you.