ar>expr</var> is ever non-<code>nil</code>, its value is immediately
returned without running epilogue code.  Otherwise, the epilogue code
is performed and <code>nil</code> is returned.

        <p>This clause cannot be used together with <code>always</code> or
<code>never</code>, because their defaults are opposed (similarly,
<code>(loop always 3 thereis nil)</code> refuses to compile in some
implementations of <code>loop</code>). 
</p></blockquote></div>

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