Recovery of Reentrant List Structures in SLIP
One consequence of the reference-count-based
space-recovery system employed by SLIP is that 
reentrant list structures are not recovered even when
explicitly erased.  LISP-like garbage-collection 
schemes are free of this impediment.  They however,
depend on being able to find and mark nodes that 
are reachable from program variables.  By tracing all
descendants from program variables may then be 
identified and collected.  The list-creating function
LIST of SLIP may be amended to mark those lists 
for which the programmer wishes to assume responsibility.
 Given this modification, a LISP-like garbage 
collector that recovers abandoned reentrant list structures
may then be appended to the SLIP system.
CACM July, 1969
Weizenbaum, J.
