Re: New undo element (fun . args)

[Richard Stallman]

    I may have misunderstood something too, but IIUC, this checks to see
    if the buffer-undo-list has not been modified since the last undo --
    by comparing the current head of buffer-undo-list with the the head of
    the buffer-undo-list at the end of the last undo as it was recorded in
    undo-equiv-table.

That is right.

    So whether that last undo command added anything to buffer-undo-list
    or not doesn't seem relevant, as the head of the buffer undo list
    should be in the undo-equiv-table in both cases,

If an undo command does not generate any undo entries, it will have to
put into undo-equiv-table a head-of-list that was produced by some
other command, which may not have been an undo command.  That is
certainly not supposed to happen.  Whether it actually leads to
incorrect results, I don't know.

    So this would be a problem only in the case where (apply ...) is the
    only action between two undo boundaries. To work around this, it seems
    to be sufficient to add just one element at the end of Fprimitive_undo
    in case the undo list hasn't changed at all.

Yes, I think so.

    BTW, I suppose that the apply undo function is not allowed to change
    current buffer.

Yes it can.  That is why it can specify a DELTA.