code: public static double angle(Quat4d q) {
  		AxisAngle4d axis = new AxisAngle4d();
  		axis.set(q);
  		return axis.angle;
  	}
docs: Calculate the rotation angle component of the input unit quaternion.
  
  @param q
  unit quaternion Quat4d
  @return the angle in radians of the input quaternion