ontaining
only unusable blocks.  This would otherwise become an issue
if there is low demand for small pointerfree objects.

<H2>Thread support</h2>
We support several different threading models.  Unfortunately Pthreads,
the only reasonably well standardized thread model, supports too narrow
an interface for conservative garbage collection.  There appears to be
no completely portable way to allow the collector
to coexist with various Pthreads
implementations.  Hence we currently support only the more
common