| 摘要 |
The cover has a frame having at least one arcuate section in which the cover sheet has a groove (15,16) with adjacent edges to receive the tensioning retainers for the cover sheet. the frame can tension the cover along a generatrix defining the arc corresponding to the groove (33). The cover sheet can be tensioned by an edge rod (17) received in the groove in the frame. The frame can have a tube (11) with two grooves.
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