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.
CACM December, 1960
Collins, G. E.
