给定一个无参数的函数 f (通常带有副作用)，返回一个调用 f 函数 n 次的惰性序列。
如果不指定参数 n ，那么函数 f 可以执行无限次。
; 测试函数
user=> (defn greet []
"hi!")
#'user/greet
; 定义一个执行 10 次 greet 的惰性序列
; 并用 take 函数取出 5 个和 10 个 greet 的执行结果
user=> (def ten-greet (repeatedly 10 greet))
#'user/ten-greet
user=> (take 5 ten-greet)
("hi!" "hi!" "hi!" "hi!" "hi!")
user=> (take 10 ten-greet)
("hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!")
; 试图取出 10086 个值，但 ten-greet 最多只返回 10 个值
; 说明取出的数量最多只能和 n 一样大
user=> (take 10086 ten-greet)
("hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!")
; 定义一个执行无限次 greet 的惰性序列
user=> (def infinite-greet (repeatedly greet))
#'user/infinite-greet
user=> (take 10 infinite-greet)
("hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!")
user=> (take 100 infinite-greet)
("hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!" "hi!")