Generational/Incremental Garbage Collector

   The generational, incremental garbage collector still is considered
   experimental, although it stands up well in some real applications.
   Here is a list of known problems:

   o  On a Sun (Sun-4/SunOS4 or 5) when compiling Elk with gcc (2.6.3
      or older), the generational garbage collector sometimes loops
      (when working in non-incremental mode).  This can be circumvented
      by compiling src/proc.c (yes, proc.c, not heap.c) without the -O
      option.  We are not sure yet whether this is a bug in Elk or in gcc.

   o  Running out of memory when expanding the heap shouldn't be handled
      as a fatal error.  Instead, the garbage collector should clean up
      and then invoke Uncatchable_Error() to return control to the Scheme
      program.

   o  The return value of ExpandHeap() is sometimes ignored.

   o  When running the program

	 (garbage-collect-status 'generational 'incremental)
	 (define (f) (make-list 10000 'a) (f))
	 (f)

      the pairs in the lists become stable quickly and aren't reclaimed,
      as the current algorithm favors heap expansions over full collections.

   o  With the same test program, the GC sometimes crashes with SIGSEGV
      after having expanded the heap to 9MB.

   o  Under HP-UX 9.0 and AIX 4.1, the GC doesn't work in incremental mode
      (a broken-heart is passed to Memoize_Frame() after an ExpandHeap()).

   o  The percentage displayed at the end of a GC run is sometimes wrong.

   o  This simple test causes a valgrind error after the first GC run:

	(define (fact x) (if (> x 1) (* x (fact (- x 1))) 1))
	(fact 100)
	(fact 1000)
	(fact 1000)