有条件地执行 LOOP 子句

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)))