filtering selection sets

I figure i am missing something obvious, but there is something i can't
figure out when filtering selection sets.

I often filter a selection set retrieved using a polygon, when i do, it is
easy to filter, such as this:
(setq ss1 (ssget "WP" ptlist (list (cons 0 "INSERT") (cons 2 rcount))))

However, if i prompt a user for a selection set, and want to reuse it over
and over, usually to count stuff, i can't filter an existing selection set
using:
(setq ss1 (ssget ss_bom (list (cons 2 fibspanlabel))))

I have this workaround, which hasn't caused any problems yet:
(command "select" ss_bom "")
(setq ss1 (ssget "p" (list (cons 2 fibspanlabel))))

I know there is a simpler and more professional way to do this, any one know
how?

I know how to step through the sset, look at each entity and ssadd it to a
second set, but i know acad has an easier way of doing it.


Thanks, wm