GSUB feature overflow

I have written a fairly complex calt feature and now MakeOTF gives me an overflow error while compiling it. How can I circumvent this?

You can try to use this:

Welcome to the club. You will have to re-factor your feature code, so that you only substitute when it’s really necessary. Perhaps you can get rid of other substitution features.

I inserted useExtension but it still causes overflow (by the way, the error message says the feature “ÿÿÿÿ” is causing the problem. I think it seems obvious in my font, but I really don’t know which feature). And I don’t know how to re-factor my code.

Rainer, shall I send the font file to you?