What does usable-duration-logs mean? The NR internals section says:
'(-3 -2 -1 0)
List of duration-logs that can be used in typesetting the grob.
I am aware the NR is reference not tutorial, but this entry has very little explanatory value. What do the logarithms do? Why do you set it to a list containing 0? These manual entries are not illuminating because it just says that the usable durations log list is a list of usable duration logs, and that they can be used in typesetting. It would be great of we could improve this entry at least. I am reminded of those all too common software user manuals that have an entry for File->Open saying open a file and that's it. Great.