sentences:  To use the same algorithm to check if the point is in the central region, generate a fictitious "x"0 = "A"/"y""1".This will generate points with "x" < "x""1" with the correct frequency, and in the rare case that layer 0 is selected and "x" ≥ "x""1", use a special fallback algorithm to select a point at random from the tail.Because the fallback algorithm is used less than one time in a thousand, speed is not essential.