title: Why does AbstractSet not include union and intersection?
body: <p>Is there a documented reason for excluding <code>union</code> and <code>intersection</code> from the defining methods of <code>abc.Set</code> and thereby from <code>typing.AbstractSet</code>? As a result, I often have to use <code>Union[Set,FrozenSet]</code> where I expected to be able to use <code>AbstractSet</code>. This is particularly puzzling given that the <a href="https://docs.python.org/3/library/typing.html#typing.Set" rel="nofollow noreferrer">docs</a> suggest giving preference to <code>AbstractSet</code> for argument type annotations.</p>