| 摘要 |
The board (1) has a central zone (9) comprising rear and front retaining zones (16, 18) and intermediate zones (15, 17, 19). Rear and front maximum width lines (W3, W4) are located between a rear end (2) of the board and rear contact line (W1) and between a front end (3) of the board and a front contact line (W2), respectively. A center (M0) between the retaining zones is located in rear and front with respect to a center (M1) between the rear and front ends and a center (M3) between the contact lines, respectively. The center (M0) is joined with the center (M2) between the width lines.
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