> >That Object is the <flag> used by WeakSet to mark empty spaces. I
> >was a bit surprised to see nils as well. Hmm...
> As I understand things, which may be not at all, the nils are put
> there by the GC when an element of a WeakArray is garbage collected.
> The flag objects are put there by the Set implementation to make
> empty spaces, so that they can be distinguished from the nils.
That's right.  When WeakSet scans to find objects, it has to treat nils
as if they were objects too.  Only when it finds the flag it knows that
the object isn't there.  For example, let's say the weak array inside
of it originally had
 weak#(... x1 x2 x3 x4 x5 x6 x7 ...)
and all the objects from x1 through x7 had the same hash.  So if you
did weakSet includes: x7 it would find x7 like a regular set would.
But since this is a weak array, things can go away.  Let's say x5 went
away.  Then you'd have
 weak#(... x1 x2 x3 x4 nil x6 x7 ...)
Finding nil while scanning doesn't mean [stop], it means [something
went away] but possibly there's more stuff behind it because you
haven't found a free spot marked by the flag object (or an object with
a different hash), so you have to continue.  Then you'd find x7.
Andres.