Comment by penteract

For that example, I think you could do

    s n a' k e -> ā
    s n ā k e' -> NULL

although that would also change "snāke" to "snāk", which is not quite what you asked for.

I don't think there's a way to do many-to-many substitution in general, although it would always be possible if you create intermediate glyphs. I believe Section 5 of http://adobe-type-tools.github.io/afdko/OpenTypeFeatureFileS... gives the full list of substitution types.