Specify whether glyph names in features.fea are design or production names

[brawer]

Could features.fea specify whether the glyph names inside the feature file are design or production names, that is whether they are the keys or the values of public.postscriptNames from lib.plist?

Currently, this is unspecified, which is rather suboptimal for a file format specification. So far, I’ve only encountered UFOs in the wild whose feature files were using design names (that is, the keys of public.postscriptNames). This probably makes sense, since designers like to work with design names. But I think it would be good if the UFO spec could state clearly whether features.fea is intended to use design or production names.