A loop is a series of shots that form a complete circle and connect back to itself. In the example, stations A3, A4, A5, and A6 form a loop.
How the Compiler Processes Loops. In a loop closure, both stations are defined. For example, the shot from A6 to A3 closes the loop and both stations are defined before the shot is processed. Since A3 is already defined, this shot defines it again. If there have been any errors in the loop, the two definitions will put A3 in different places. The program calculates the difference between these two locations and this measures the amount of error in the loop.