Should processors work with references? I would think that they should, but the following fails. (My backmatter page numbers need to be marked to distinguish them from bodymatter pages. I can automate applying a processor based on the location for index register entries, but not for references.)

\defineprocessor[xx][style=slanted,after=n]
\starttext
\section[xxx]{Processor testing}
Why cannot a processor be applied to a reference? \at[xx->xxx]
\stoptext