okes garbage collector explicitly, returning a list of two integers,
numbers of free words and total words (not bytes) allocated in the heap.
<B>SYS:*GC-HOOK*</B> is a variable to hold a function that is called upon the
completion of a GC.  The hook function should receive two arguments
representing the sizes of the free heap and the total heap.

<P>
If "fatal error: stack overflow" is reported during  execution,
and you are convinced that the error is not caused by a infinite loop
or recursion, 
you can ex