Get the subexpressions of a live script

I want to retrive the sigma_n subexpressions in the live script. Is there a way to do it. I searched but couldnt find it.

Moreover some of the this sigma_n subsexpressions i want to to equalize to zero in the main expression dUs2. If after i find the sgma_n subsepressions can i make subs(dUs2,sigma1,0) or something like that.

Thanks for the help.

dUs2 =

[Walter Roberson]

It is not clear what you are asking for, but I speculate that you might be looking for coeff()