Dmitry

So, you are suggesting a runtime option, not another type trait?

Do I understand it correctly, that you want to both allow missing fields, and allow extra fields? Or just the latter?

To allow missing fields our standard solution is to use optional. Would this work for you?

If my PR #296 is merged, then (I think) one could do ``` lib mylzma : lzma.cpp ; external.configure lzma : lzma=mylzma ; ```