"Plan B" in GlyphsData decomposition

Glyphs
1

I want to have more than one decomposition rule in GlyphsData.xml.

For example, quotedblbase has “quote, quote” as decomposition value, but I want to use “quotesinglbase, quotesinglbase” as primary rule and “comma, comma” as a secondary option.

I can just think of this one particular case for now, but how else would this be useful?

2

This specific use case is not strong enough for me to add that feature. If there are stronger examples I have a look.