Computational behavior theory and cultural evolution
Put 1/2 cup water, the lemon rind and the sugar in a pan, dissolve the sugar and let the mixture warm up and boil for 2-3 min. In the meanwhile, whisk the egg whites in a bowl until firm. Pour all the ingredients in the ice-cream maker bowl and, if you have time, let cool. The egg whites will float on top but will get mixed in eventually.
Note: There are recipes around with as much as 6 times more sugar than I use. Besides the general inflation of sugar content in food, one reason is that people try to balance the sourness of the lemon with more sugar. That does not really work, however, and you end up with something that is both very sour and very sweet. Adding salt neutralizes some on the sourness. In fact, you can experiment with even more salt than indicated above.
drghirlanda 
I tried to make this with lime, too, but it came out somewhat bitter and sour. I suspect the rind is to blame for the bitterness. One could use more sugar (say, 3/4 cup) and add the rind to the water+sugar mixture and then strain it out.
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