Thanks for the help. I 'discovered' the isinstance method in the ZAPI Reference of your book, which is just what I needed.
Note that with isinstance, you check for a certain implementation, not for functionality. In general, that is not preferrable because implementations may change, interfaces usually don't.
So, for filtering, it's preferrable to use ISomeInterface.providedBy(obj)
I put the filter method in the custom content class rather than a separate view class. It all seems to work so I am progressing again.
Putting display filtering in a content class is not quite a kosher way to go. You should filter in the browser view.
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