You are given the following information, but you may prefer to do some research for yourself.
- 1 Jan 1900 was a Monday.
- Thirty days has September, April, June and November. All the rest have thirty-one, Saving February alone, Which has twenty-eight, rain or shine. And on leap years, twenty-nine.
- A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
周日计数
下列信息是已知的，当然你也不妨自己再验证一下：
- 1900 年 1 月 1 日是周一。
- 三十天在九月中， 四六十一也相同。 剩下都是三十一， 除去二月不统一。 二十八天平常年， 多加一天在闰年。
- 闰年指的是能够被 4 整除却不能被 100 整除，或者能够被 400 整除的年份。
在二十世纪（1901 年 1 月 1 日到 2000 年 12 月 31 日）中，有多少个月的 1 号是周日？
同样，没什么好说的：
(defun eu19-leap-p (y)
(or (and (zerop (% y 4)) (not (zerop (% y 100))))
(zerop (% y 400))))
(defvar eu19-month
[31 28 31 30 31 30 31 31 30 31 30 31])
(defvar eu19-leapm
[31 29 31 30 31 30 31 31 30 31 30 31])
(named-let f ((year 1901) (day 366) (cnt 0))
(cond
((= year 2001) cnt)
(t (cl-loop for a across (if (eu19-leap-p year) eu19-leapm eu19-month)
do (if (= (% day 7) 0)
(cl-incf cnt))
do (cl-incf day a))
(f (1+ year) day cnt))))
;; 171