Fast way to determine if there is only one reference to an object?

["Duncan Cross" <duncan.cross@...>]

Hi L-L,

Is there a good way (through the C API, presumably) to determine
whether there is more than one reference to a given object (table,
full userdata, thread, function)? I don't need to know exactly how
many there are, beyond the dinstinction between "one" and "more than
one", and I even don't mind the possibility of occasional false
positives (returning "more than one" when there is actually only one),
but false negatives would be a problem.

-Duncan