If you change the statement to AND, User is presented with a menu listing thirteen printers numbered 0 to 12.

You can select 0 or 1 or 10 or 11 or 12, and the Gosub add_printer statement works, as it should.

However, if you select 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9, all of which should be valid choices, you get the Invalid choice message printed.

AArrrrgggghhhhhh