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 integer type.
	</li>
	<li>An element without a key uses the previous element's index plus one.
	    If the first element has no key, its index is zero.

It allows a counting semaphore to be modeled by a buffered channel:
the number of items in the channel corresponds to the number of active uses,
the capacity of the channel corresponds to the maximum number of simultaneous uses,
sending an item acquires the semaphore, and receiving an item releases
the semaphore.
This is a common idiom for limiting concurrency.
</p>

<p>

	    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.