The insertion part (23) has a top surface (25) and a bottom curved surface (26) whose vertical size at the base end of the insertion part is greater than the vertical size at its point. The insertion opening (32) has a top surface which can engage with the top surface of the insertion part, and a bottom surface (34) which can engage with the base end of the curved surface. The top surface has a top protuberance (29). The top surface of the insertion opening has a top indentation (39) engaging with the protuberance. The outer periphery of the handle (22) is greater than that of the insertion part, to form a step and a top corner (30) between the handle and insertion part.
