Interface of the set classes

[Terry Reedy]

"Pierre Barbier de Reuille" <pierre.barbier at cirad.fr> wrote in message 
news:41824654$0$1779$636a15ce at news.free.fr...
> For me, sets are first sequences.

In mathematics, a set is determined by its members.  Period.  The concept 
'ordered set' -- a set with an order relation on its members, where a 
relation is a set of ordered pairs -- is a specialization (and 
complexification) of the concept 'set'.  If some other language misuses 
'set' to mean 'ordered set', you have been done a disservice by that 
language.

Terry J. Reedy