4clojure 题解 Count a Sequence

统计集合元素的数量不允许使用count操作

Write a function which returns the total number of elements in a sequence.

	(= (__ '(1 2 3 3 1)) 5)
	(= (__ "Hello World") 11)
	(= (__ [[1 2] [3 4] [5 6]]) 3)
	(= (__ '(13)) 1)
	(= (__ '(:a :b :c)) 3)

用递归搞定

(fn d [seq]
   (if (empty? seq)
     0
     (+ (d (rest seq)) 1)))