common-lisp 有条件地执行LOOP子句
示例
LOOP有自己的IF语句，可以控制子句的执行方式：
(loop repeat 1000
for x = (random 100)
if (evenp x)
collect x into evens
else
collect x into odds
finally (return (values evens odds)))在IF主体中组合多个子句需要特殊的语法：
(loop repeat 1000
for x = (random 100)
if (evenp x)
collect x into evens
and do (format t "~a is even!~%" x)
else
collect x into odds
and count t into n-odds
finally (return (values evens odds n-odds)))