999 1 2 3)
: (assoc 'u '((999 1 2 3) ("b" . 7) ("ok" "Hello")))
-> NIL
</pre></dd>

<dt><a id="at"><code>(at '(cnt1 . cnt2|NIL) . prg) -> any</code></a></dt>
<dd>Increments <code>cnt1</code> (destructively), and returns <code>NIL</code>
when it is less than <code>cnt2</code>. Both <code>cnt1</code> and
<code>cnt2</code> should be positive. Otherwise, <code>cnt1</code> is reset to
zero and <code>prg</code> is executed. Returns the result of <code>prg</code>.
If <code>cnt2</code> is <code>NIL</code>, nothing 