functions like math.atan(y / x), except it also takes into account the quadrant of the angle and so doesn't have a limited range of output.
atan( 1 ) and atan2( 1, 1 ) are both math.pi / 4
atan2( -1, -1 ) equals to ( (-3) * math.pi ) / 4
0.7853981633974483
-2.356194490192345