Many thanks to RagDyer, though trying the full 702 range (a to zz) makes
Excel burp: it appears to accept the full range, but when I use it in Fill
Series...Autofill it goes only as far as yy, then restarts at a,b.... I can
live with that, though if anyone can explain why Excel (2000 Professional)
uses only the first 675 items of an instructed sequence of 702 I'd love to
know. (? 26^2 -1 ?)

I couldn't get Excel to accept differentiated cases as separate lists,
either; but Autofill gives lower or upper according to the case of the head
cell.