that it is connected to a portion protruded by a predetermined length from the rear edge of the upper end portion of the second part 520 the connection portion can be thinly formed to be folded at this time the connection portion forms the first curved part 581 since the first curved part 581 is provided at the rear portions of the first and second parts 510 and 520 the first part 510 can be relatively rotated clockwise with respect to the second part 520 based on the first curved part 581 see fig6 the front edge of the lower end portion of the second part 520 is protruded by a predetermined length downwardly such that it is connected to the portion protruded by a predetermined length from the front edge of the upper end portion of the third part 530 at this time the connection portion forms the second curved part 582 since the second curved part 582 is provided at the front portions of the first and second parts 520 and 530 the second part 510 can be relatively rotated counterclockwise with respect to the third part 530 based on the second curved part 582 see fig6 in summary the first curved part 581 is.