Sets and predicates

Set theory doesn't make any distinction between sets defined extensionally (sets) or sets defined intensionally (predicates). Here are some sets:

#{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
#{0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20}

Here are the same things as predicates:

(fn [n] (<= 0 n 10))
(fn [n] (and (even? n) (<= 0 n 20)))

Some means must be developed to deal with these two different presentations of classes.