elisp question: how to walk through a variable like special-display-buffer-names

[leo]

hi there

i want to walk through the list in special-display-buffer-names (and
special-display-regexps), in order to find out if the left parameter is
set in the contained alist.

emacs itself m,ust do similar things after creating frames, but i
couldn't find a hint in emacs' lisp-files.

so, are there some ready made functions to walk through "lists of elements
which are possbily again lists containing a car (buffer-name) with an
alist (frame-parameters)."

even harder and more interesting for the special=display-regexps case...

thanks in advance, leo