How to define multipart bibliography entries in amsrefs?

I would like to get something like in How can I adjust the notation in the bibliography?, but for amsrefs. IOW, I may have a "single" entry in the bibliography, like [1], or a "group" of entries, like [2a], [2b], [2c]. How do I achieve this with amsrefs? (Alternatively, I could convert my bibliography to BibLaTeX – I don’t want to, since I have almost 300 entries there and it’s a bit of work, but it’s better than nothing. Using BibTeX is out of question for me.)