| 摘要 |
The sheet material is preferably waterproof. It is in two similar halves lying flat against each other. Each of them has a bottom edge (2), from which side edges diverge upwards (3,4), and the halves are joined together along these bottom and side edges. The shape of the side edges is such that the top part of the half increases in width faster than the bottom part does. The side edges (3,4) can include an angle of roughly 60 deg. with the extensions of the bottom edge.
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