Hi!
Thanks to help here, I now know how to set a cell to
allow the user to select items from a list elsewhere
in a spreadsheet. Data->Validation->List works great!

But now I'd like to expand that. I want to make that
cell be a dropdown list SOMETIMES, but other times
force a specific value (or nothing) into it, all based
on the value of another cell.

If I put in a validation rule, there's no provision
for doing an "if(x,y,z)" where either y or z makes a
dropdown list while the other one does not.

How can I do this?

Mike