Say the list is 7 columns wide. If I put "A" in B2, I want to be able to
input data in C2 thru G2. But, if I put "B" or "P" in B2, I want cells C2, E2
and F2 to allow no data to be entered. If the B2 entry is changed to "A", all
cells C2 thru G2 become available for entry again.