Cook all in a medium sized pot over medium heat until thickened. If the rhubarb is fresh you will need to add a little water (1 cup) so it won't stick to the pot. If it is frozen and thawed it will have plenty of water. Be sure to check it often and stir so it doesn't stick to the pan. You may need to turn the heat to low once it starts to boil. This should cook down to about 3 cups. If this is not sweet enough you may need to add more sugar or a no calorie sweetener to taste.