Edit your selections
Select multiple things, run this command via the command pallet:
The command then keeps every 'a' selections, starting at the first, and de-selects the rest.
Or you can give the period an offset like: an+b or an-b and the command will keep every 'a' selections, starting with offset 'b'.
The 'n' in the input does not necessarily need to be an 'n', it can be any letter [a-zA-Z].
The offset can also be negative.