restrict kern groups to those beginning with public.kern{1,2} prefix?

The UFO3 spec is clear about this point:

1. Kerning group names must begin with the appropriate prefix.

http://unifiedfontobject.org/versions/ufo3/groups.plist/

However, in our kernFeatureWriter we consider a group as a kerning group if it used in kerning.plist, either as left, right or both position.

https://github.com/googlei18n/ufo2ft/blob/b3cdd768f7d67d5f8a7e74b57fd4e28c97b40042/Lib/ufo2ft/featureWriters/kernFeatureWriter.py#L262-L277

Why is that? Is for backward compatibility with some previous UFO1-2 conventions?

Should we bother restricting the valid kerning groups to those with the right public.kern prefix only when the UFO formatVersion is 3?

Actually, i’m not even sure defcon provides a public api to retrieve the original UFO’s format. The UFOReader knows that, of course, but defcon Font hides it in a _ prefixed attribute (_ufoFormatVersion).

[adrientetar]

Sure. defcon is always ufo3 internally.

[anthrotype]

the only supported kerning groups are those prefixed with public.kern1. and public.kern2. as in UFO3 spec.