Thunk note

[Hugh Hoover <heh>]

Who, in FM, is responsible for handling Undo?

obviously, the appropriate controller will take part, but is it reasonable
for the Thunk that actually performed the action to be able to reverse
the action?  If not, who will maintain the necessary state to accomplish
the undo.
  Note that this is more general than the trivial Xanadu undo (move the bert),
and will have to be able to undo things having nothing at all to do with
the backend.
--Hugh