I'm currently using the following snippet of c# to cause a game object to turn to face the player:

void Update()
{
     Vector3 offsetPosFromPlayer = thePlayer.transform.position - transform.position;
     Quaternion rotToPlayer = Quaternion.LookRotation( offsetPosFromPlayer );
     transform.rotation = Quaternion.Slerp( transform.rotation, rotToPlayer, (Time.deltaTime * yawRotationSpeed) );
}

So far, it's working great. However, I'd like to clamp the rotation so that the object can only turn to a variable degree on either side. Say, for example, 180 degrees, so that if the player is behind the object it cannot turn to look at him.

I'm not sure where to insert the clamp, however, given that the above code deals with quaternions. Any suggestions? Am I going about this the wrong way?