九连环

Core Structure and Numbering: Sword Frame: A long, narrow metal plate or handle. Nine Rings: Arranged sequentially on the sword frame. For ease of explanation, we usually refer to the ring closest to the opening of the sword frame (the easiest end to remove) as Ring 1, and so on up to Ring 9. Basic Operating Rules: All variations of the nine-ring combination strictly adhere to the following two core rules. We call inserting a ring into the sword frame "up," and removing it from the sword frame "down." Privilege of Ring 1: Ring 1 can be freely "up" or "down" in any situation. Conditions for Other Rings: If you want to "up" or "down" Ring $n$ (where $n \ge 2$), two conditions must be met simultaneously: Ring $n-1$ must be on the sword frame. All rings before Ring $n-1$ (i.e., Rings 1 to $n-2$) must not be on the sword frame. For example: If you want to remove (or put on) the 4th ring, then the 3rd ring must be on the sword frame, and the 1st and 2nd rings must already be removed.