Checking if a string is null

[Manfred Nowak]

Bruno Medeiros wrote

> What do you mean by "I believe that types exist for which such a 
> relation is not uniquely determined" ?

"most discriminating" implies an ordering on the set of equivalence 
relations. For some types this ordering might be a partial ordering.

If such types with partial orderings on their equivalence relations 
exist, then 
1 there might be no canonical minimal element, or
2 there is a canonical minimal element, but it is not "possible", which
: makes this case equivalent to the one above.

If there is no minimal element then there exist (with respect to 
discrimination) at least two uncomparable equivalence relations.

-manfred