a Protocol</h3>
<p>When implementing a custom type that conforms to the <code>SetAlgebra</code> protocol,
you must implement the required initializers and methods. For the
inherited methods to work properly, conforming types must meet the
following axioms. Assume that <code>S</code> is a custom type that conforms to the
<code>SetAlgebra</code> protocol, <code>x</code> and <code>y</code> are instances of <code>S</code>, and <code>e</code> is of
type <code>S.Element</code>---the type that the set holds.</p>
<