t ["(",x,"+",y,")"]) "0" (map show [1..13])"(1+((2+3)+(((4+5)+(6+7))+((((8+9)+(10+11))+(12+13))+0))))"无限树状折叠，可以用Haskell的通过埃拉托斯特尼筛法的递归素数生成来演示：primes = 2 : _Y ((3 :) . minus [5,7..] . foldi (\(x:xs) ys -> x : union xs ys) [] _Y g = g (_Y g) -- = g . g .