Parameter expansion only to existing filenames

[Vin Shelton <shelton@icd.teradyne.com>]

I was recently re-reading the Zsh User Guide (Thanks, Peter!!) and I
started a few experiments.  One need I have that is as yet unfulfilled
is to combine a list of directories and list of files such that only
the ones which exist get listed.  Here's what I mean:

zsh -f
denver% echo $ZSH_VERSION
4.1.1
denver% mkdir a b c
denver% touch a/x b/y c/z
denver% dirs=(a b c)
denver% files=(x y z)
denver% print $^dirs/$^files
a/x a/y a/z b/x b/y b/z c/x c/y c/z

What I'd like to generate is:
a/x b/y c/z

GLOB_SUBST doesn't help:

denver% print $^~dirs/$~^files
a/x a/y a/z b/x b/y b/z c/x c/y c/z

Is this because there's are no patterns to glob?

Is there a clever way to generate the list I want?

Thanks in advance,
  Vin