If you have four options, each of which must be in one of two possible states (ABSENT or PRESENT) then you can easily map it to a four-bit binary encoding with 16 (rather than just 15) possible states, right? 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111