. . "FIG. 7A is an explanatory drawing of a connecting portion 24B. FIG. 7B is an explanatory drawing of a connecting portion 24C. FIG. 7C is an explanatory drawing of a connecting portion 24D. For example, although each of the first and second tapered surfaces 21 a and 31 a is flat in the foregoing embodiment, the connecting portion 24B that is formed of curved surfaces and makes acute angles with the" . .