(defun euler-problem-1 ()
  (let ((threes (loop for x from 1
                      for y = (* x 3)
                      while (< y 1000)
                      collect y))
        (fives (loop for x from 1
                     for y = (* x 5)
                     while (< y 1000)
                     collect y)))
    (reduce '+ (sort (remove-duplicates (append threes fives)) #'<))))