Thanks Misha & Matthias. I now get what's going on. The mention of the word "dictionary" revealed it all. I've spent the last 7 years programming in C++, and had dynamic_cast<> firmly fixed in my head. I totally forgot that Fergus Henderson and I independently reinvented dictionary passing for the implementation of Mercury circa 1995. :-) I also realized that the concrete signatures from my actual application don't quite satisfy the requirements anyway, and that the problem evaporates when I get the signatures right.:-) T.