nano editor on OS X - How to type M-U for undo? : <p>I am using nano 2.5.3 on OS X Yosemite, and I see commands at the bottom such as:</p>

<p>M-U Undo
M-E Redo</p>

<p>So far, I have not been able to figure out which key or keys that <code>M</code> is referring to. What would be <code>M</code> on OS X?</p>
