| 摘要 |
A series of U-shaped frames have their web and legs of progressively greater length. A smaller-size frame fits between and is pivoted to the free ends of the legs of the next larger-size frame. When the device is extended, each frame projects outwardly from an adjacent frame. In folded position, each frame nests within the next larger frame. At least some of the frames have ground-engaging teeth and the smaller-size frame has tire-engaging teeth.
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