its position in the array.
	</li>
	<li>An element with a key uses the key as its index. The
	    key must be a non-negative constant
	    <a href="#Representability">representable</a> by
	    a value of type <code>int</code>; and if it is typed
	    it must be of <a href="#Numeric_types">integer type</a>.
	</li>
	<li>An element without a key uses the previous element's index plus one.