problem in MPU6050_6Axis_MotionApps20.h

In this file I found that this code:

uint8_t MPU6050::dmpGetEuler(float *data, Quaternion *q) { data[0] = atan2(2*q -> x*q -> y - 2*q -> w*q -> z, 2*q -> w*q -> w + 2*q -> x*q -> x - 1); // psi data[1] = -asin(2*q -> x*q -> z + 2*q -> w*q -> y); // theta data[2] = atan2(2*q -> y*q -> z - 2*q -> w*q -> x, 2*q -> w*q -> w + 2*q -> z*q -> z - 1); // phi return 0; }

And I cannot find any DCM, quaternion to verify the correctness of this code . It is totally different from any formular I used to learn. Are there anybody have a solution for this. Thanks in advance