cleanup(n)  sets the cleanup period to its integer argument n and returns the
	    former value of the cleanup period.  cleanup with no arguments
	    just returns the current cleanup period.

	    The cleanup period is the number of loop iterations which must
	    occur before temporary data is cleared.  The default value is 10.
	    Increasing the cleanup period causes loops to execute faster at the
	    expense of more memory usage.  If you have large amounts of
	    temporary data and run out of memory during loop execution, try
	    cleanup(0).