Ok, thanks for all the advice from everyone. :-)

Essentially, I wish to hash the pointer value of the reference itself. So, if 
a reference were reassigned, then the hash would be different. Two identical 
copies of an object would have different references and, consequently, would 
give different hashes (i.e. a != b is true where a and b are 'a ref).

The reason I want to do this is to spot duplicate uses of a single object 
(multiple references to it) but without confusing this with different uses of 
identical objects (i.e. where "=" would give true but "==" would not).

I can think of a couple of alternative approaches. I could implement a naff 
kind of set as a list which used "==" to test for equality. Or, I could loop 
through all of the references to the objects, marking unmarked objects with a 
new identifier (an int, say) and then insert them all into a Hashtbl on the 
basis of their identifiers. I just thought hashing refs themselves would be 
simpler.

I think I can do this given Alain's code though, thanks!

Cheers,
Jon.

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