assq_no_quit... huh?

[Po Lu]

What's the meaning of this code?

          if (is_tty_frame (f))
            {
              /* Ignore all invisble tty frames, children or root.  */
              if (!FRAME_VISIBLE_P (root_frame (f)))
                continue;

              /* Remember tty root frames which we've seen.  */
              if (!FRAME_PARENT_FRAME (f)
                  && NILP (assq_no_quit (frame, tty_root_frames))) <===
                tty_root_frames = Fcons (frame, tty_root_frames);
            }

No alist is being constructed and consequently assq_no_quit always
returns nil.