> What I meant actually, was a «physical compare». I found out that 
> this (crude) C function works :
> 
> value phys_comp(value a, value b)
> {
>  if(a<b)
>    return Val_int(-1) ;
>  else if(a>b)
>    return Val_int(+1) ;
>  else 
>    return Val_int(0);
> }

No, it does not:  the GC can copy heap blocks and change the relative
ordering of their memory addresses.  You really need to pair your
functions with unique identifying integers.

- Xavier Leroy
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