_arguments -s -w and sets

[Eric Cook <llua@xxxxxxx>]

I've noticed when trying to complete multiple options in one word,
options that should be ignored via sets are still completed.

with:
compdef '_arguments -s -w : -a - set1 -b - set2 -c' foo

foo -c <tab> # completes -a alone.
but
foo -c<tab> # offers both -a or -b

Is that intentional and if so, is there a way to get the desired effect
of ignoring -b?