> Also if the proposed change to `window-deletable-p` is adopted, then a
 > grep on `window-deletable-p` also reveals that, for consistency,
 > `delete-windows-on` and even `calendar-exit` in `calendar.el` should
 > be modified similarly to `window--delete`.

This means that we should do the entire fix within 'window-deletable-p',
right?  That's not an internal function, so we can, in all conscience,
supply an abnormal hook as in the roughly tested patch attached.

martin