Yeah, that&#x27;s a good question. Even though the mu core is at the top directory, there are a couple of apps in sub-directories of their own. To run a single-file mu program you say:<p><pre><code>  $ .&#x2F;mu factorial.mu
</code></pre>
To run a more complex app, you just give a directory name rather than a filename:<p><pre><code>  $ .&#x2F;mu edit
</code></pre>
Files inside the directory are loaded in numeric order just like at the top level. You can also run just a subset of the layers for the editor:<p><pre><code>  $ .&#x2F;mu test edit&#x2F;001*
  $ .&#x2F;mu test edit&#x2F;00[12]*
  $ .&#x2F;mu test edit&#x2F;00[1-3]*
</code></pre>
(Notice how the number of tests&#x2F;dots grows at each layer.)<p>Since the layers are just regular files there&#x27;s nothing stopping you from rewriting history as much as you want. That ability was precisely what I built them for.<p>I&#x27;ve only recently started using directories, so I&#x27;m sure there&#x27;s stuff here I haven&#x27;t considered. Feedback most welcome.