Part of the System -> MorphicExtras dependency stems from references
to CommandHistory. Now it seems that CommandHistory has almost nothing
to do with MorphicExtras. The only thing that ties it to that package
is CommandHistory class >> #resetAllHistory, which has the line

MorphExtension withAllSubclassesDo:
    [:mexc | self forgetAllGrabCommandsFrom: mexc someInstance]

So it occurs to me that CommandHistory is useful beyond the context of
a MorphicUI, and hence perhaps we can resolve this territory dispute
thusly:
* move CommandHistory to somewhere in System (System-Support, perhaps), and
* leave resetAllHistory in MorphicExtras, perhaps with
#forgetAllGrabCommandsFrom:.

This doesn't fully resolve the issue, because SmalltalkImage >>
#zapAllOtherProjects calls #resetAllHistory (among other
dependency-on-MorphicExtras-causing actions), but does snip one more
thread.

What do you think?

frank