(define (ncsubseq lst)
  (let recurse ((s 0)
                (lst lst))
    (if (null? lst)
        (if (>= s 3)
            '(())
            '())
        (let ((x (car lst))
              (xs (cdr lst)))
          (if (even? s)
              (append
               (map (lambda (ys) (cons x ys))
                    (recurse (+ s 1) xs))
               (recurse s xs))
              (append
               (map (lambda (ys) (cons x ys))
                    (recurse s xs))
               (recurse (+ s 1) xs)))))))