main/src/georegression/geometry/UtilLine2D_F64.java
public static double acuteAngleN( LineGeneral2D_F64 a , LineGeneral2D_F64 b ) {
		double value = a.A*b.A + a.B*b.B;
		if( value < -1.0 ) value = -1.0;
		else if( value > 1.0 ) value = 1.0;
		return Math.acos(value);
	}
public static double acuteAngleN( LineGeneral2D_F64 a , LineGeneral2D_F64 b ) {
		double value = a.A*b.A + a.B*b.B;
		if( value < -1.0 ) value = -1.0;
		else if( value > 1.0 ) value = 1.0;
		return Math.acos(value);
	}
Returns the acute angle between the slope of two lines and assumes that the lines have
been normalized such that A*A + B*B = 1.  This avoids the need to compute the square root
twice.  Lines do not need to ever intersect.  Found using the dot product.

@param a (input) normalized line
@param b (input) normalized line
@return acute angle in radians