| http://www.w3.org/ns/prov#value | - The holding members 17, 17 and the opposed walls 14, 14 are formed in such a manner that the distance or gap G (shown in FIG. 6) formed between the holding members 17, 17 and the opposed walls 14, 14 becomes slightly narrower toward upward in the plane including the contacting surface of the opposed walls 14, 14 as indicated in FIG. 10.
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