atan2(y,x)                   arc-tangent

    Return arc-tangent of y/x,  -. to .. If  DEGREES TRUE return result  in
    degrees, else return result in radians.

    DEGREES = TRUE;
    atan2(1,1) == 45
    atan2(-1,0) == 180
    DEGREES = FALSE;
    atan2(-1,0) == pi