Why is this not a valid usage of byref?
<pre><code>let iter2D (map: 'T byref -&gt; unit) (arr: 'T[][]) =
    for y = 0 to arr.Length - 1 do
        let row = arr.[y]
        for x = 0 to row.Length - 1 do
            let mutable elem = arr.[y].[x]
            map &amp;elem
</code></pre>

<p>The last line has: "The address of the variable 'elem' cannot be used at this point." What's wrong?</p>