Roger Govier wrote...
....
=SUM(First:Last!A1)
Of course, the sheet with the formula must be outside of the "sandwich"
created by First and Last.
....

No it doesn't. The only restriction on this particular formula is that
it can't be in cell A1 in any of the worksheets between First and Last,
inclusive, without causing circular recalc. It'd work just fine entered
into any other cell.