That's why I was careful to refer to them as 11-Pebble and 13-Pebble Favorers. They do value other sizes of pebble heaps, 11 and 13 are just the numbers they do most frequently. Or perhaps 11 and 13 are the heaps they like making in their personal time, but they like larger prime numbers for social pebble-sorting endeavors. The point is, I said they "favored" that size because I wanted to make sure that the ease of sorting the piles didn't seem too relevant, since that would distract away from the central metaphor.