[plt-scheme] Is there a set data-type?

David Van Horn wrote:
> 
> Uh, how do you represent the set of all even numbers, for example?
> 
> This is an exercise we give our first semester undergraduates here at 
> Northeastern.  I'll give you a hint: you don't need lists or hash 
> tables, go back to the mathematical foundations and think characteristic 
> functions.
> 
> Now you do union, intersection, setminus, etc.
> 

Good example... in fact, I didn't even thought about it when I 
implemented this... ahah Funny. Thanks for the counterexample.

I guess the characteristic function approach involves using lambdas to 
represent a set... :-/ I am worried with efficiency but I will give it a 
try.

> David
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