Clean up not only removing ineffective kerning values?

Using the Kerning-Window, I copy-pasted Groups and Kerning pairs from one of my older Glyphs-Files to a new one. Because the new file contains less characters than the new one, I used the “Clean up” button at the lower right corner of that window. This resulted in 30% less lines in the Kerning Window. As I was curious to see what had happened, I used “Edit –> Compare Fonts” and was surprised to see that “Clean up” not only “removes ineffective kerning values, e.g., leftovers like kerning entries of letters that are not in the font anymore” but also kerning pairs that are valid. Is this a bug or what is happening here?

I also had this impression, but did not investigate it further for lack of time but decided not to touch that button anymore for the time being. My impression was that kerning exceptions were purged in the process.

Thanks, that could make sense, but removing exceptions should rather be an option, I think. Especially as Compress is said to do such a thing. And even for “Compress” I would like to be able to choose between “Compress and remove Exceptions” and “Compress and keep Exceptions”.

“Clean Up” did not remove exceptions. I still have no idea what “Clean Up” really does …

After some checking it seems that a certain amount of differences was caused by the fact that in one of the two Glyph files the kerning groups were named »guillemot«, after the old (mistaken) PostScript standard as common in FontLab 5 and older. In the other Glyph file both the characters and the groups were named »guillemet« i.e. correctly. Furthermore some of the groups (the ones with acute in the name) had been reorganized and named differently. Still it would make life easier when ‘Clean Up’ would be specified (more) precisely somewhere in the manual.

Handbook, section 9.2.6, page 119:

After removing glyphs or after importing, invalid or impossible kerning pairs may be left over. You can remove them with the Clean Up function from the gear button in the Kerning window.

Removing exceptions is called ‘compressing’. For better exception-cleaning options, see the mekkablue scripts and the kerning-related filter plug-ins, all available in Window > Plugin Manager.

It might be a problem with differences in class names.