A vent stack screen for portable toilet holding tanks is provided in the form of a cap designed to fit over the open end of an open air vent stack extending from the holding tank through the roof of a portable toilet. A transport mechanism receives the documents from the input receptacle and transports the documents past a full image scanner and a discrimination unit. The network unit has a selector for simultaneously performing a plurality of data transfers which are each made from a register file of one processing element to an operation unit of another processing element. A suction cup may also be employed within the receiving portion of the prosthetic limb for aiding in retention of the residual limb.