Abstract: An elongated hollow rear member has front and rear ends with an open front end and a top surface which extends inclinedly downward from its rear end to its front end. An elongated hollow middle member has front and rear ends with an open front end and a top surface which extends inclinedly downward from its rear end to its front end, the maximum height of the top surface of the middle member being less than the minimum height of the top surface of the rear member. A first device cooperates with the rear and middle members to establish a slideable interconnection therebetween. The first device has an extended position at which the rear end of the middle member is disposed adjacent but in front of the front end of the rear member and has a withdrawn position at which the rear end and of at least a substantial adjacent portion of the middle member is disposed within the front end and a substantial adjacent portion of the rear member.