A function designator for a function of one argument, or
If blocking-gen-num-func is a function designator it should take two arguments: the generation number and, if do-gc in the last call to set-blocking-gen-num was a number, the number of copied segments. It is called whenever the blocking generation is garbage collected automatically. If blocking-gen-num-func is
nil, then this callback is switched off.
If other-func is a function designator it should take one argument, the generation number that was garbage collected. It is called whenever an automatic GC occurred and blocking-gen-num-func was not called, either because the blocking generation was not garbage collected, or because blocking-gen-num-func was passed as
nil. If other-func is
nil (the default) then this callback is switched off.
LispWorks User Guide and Reference Manual - 20 Sep 2017