Roger Govier wrote...
True, Harlan, but as a generality I think it wiser to suggest that users
keep the sheet with the summation formula(e) outside the "sandwich" to avoid
any chance of circular referencing.
Hopefully it avoids the re-posts with " it doesn't work...."
....

There's a difference between 'must be' and 'should be'. If you mean
'should', use 'should'.