$//$ Mask for extracting a section from the least significant bits of an aisle.

$//($aisle & SECTION$\_$MASK$)$ should preserve a section$\text{'}$s worth of bits at the

$//$ lower end of the aisle and set all other bits to $0.$ This is essentially

$//$ extracting section $0$ from the example aisle shown above.

#define SECTION$\_$MASK $0$x$????$