title: How to extract osm poi categories
body: <p>This is an example of osm xml: </p>
  
  <pre><code>  &lt;node id="1000" ...&gt;
  ...
      &lt;tag k="shop" v="supermarket"/&gt;
    &lt;/node&gt;
  
    &lt;node id="26999673" ...&gt;
  ...
      &lt;tag k="public_transport" v="station"/&gt;
      &lt;tag k="railway" v="station"/&gt;
      &lt;tag k="train" v="yes"/&gt;
    &lt;/node&gt;
  </code></pre>
  
  <p>I know that "shop", "railway", "station"  are poi categories, because I am human. But how can I extract them by script?  </p>