A recess has a proximal-end-side surface and a distal-end-side surface. When an intersection between a straight line included in the proximal-end-side surface and a straight line included in the distal-end-side surface is a base point, a line passing through the base point is a base line, a line between two claw portions, passing through an end point of the recess and orthogonal to the base line is an orthogonal line, the angle α made between the base line, and the straight line included in the distal-end-side surface is greater than 0 degrees and less than 90 degrees, the angle β made between the orthogonal line, and the straight line included in the proximal-end-side surface is greater than 0 degrees and less than 90 degrees, and the length d from the base point to the orthogonal line is greater than 0.