Why doesn't this set comprehension work?
===

In Python, given this set

    mylist = [20, 30, 25, 20]

Why does this set comprehension not work?

    >>> {x for x in mylist if mylist.count(x) >= 2}
      File "<stdin>", line 1
        {x for x in mylist if mylist.count(x) >= 2}

Thank you.