106	 A Method for Overlapping and Erasure of Lists	 An important property of the Newell-Shaw-Simon scheme for computer storage of lists is that data having multiple occurrences need not be stored at more than one place in the computer. That is lists may be overlapped. Unfortunately overlapping poses a problem for subsequent erasure. Given a list that is no longer needed it is desired to erase just those parts that do not overlap other lists. In LISP McCarthy employs an elegant but inefficient solution to the problem. The present paper describes a general method which enables efficient erasure. The method employs interspersed reference counts to describe the extent of the overlapping.
