How to make something only on a part of a signal (let say negative part) in Simulink?

Two sine waves (one shifted for 90 degrees) pass into an atan2 block which gives me a signal which goes from -pi to +pi. I would like to add 2pi only on the negative part. Which block should I use after atan2 block?

Thanks

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/simulink/slref/if.html

http://www.mathworks.com/help/dsp/ref/unwrap.html