R rvest: could not find function "xpath_element"
<p>I am trying to simply replicate the example of <code>rvest::html_nodes()</code>, yet encounter an error:</p>

<pre><code>library(rvest)
ateam &lt;- read_html("http://www.boxofficemojo.com/movies/?id=ateam.htm")
html_nodes(ateam, "center")
</code></pre>

<blockquote>
  <p>Error in do.call(method, list(parsed_selector)) :    could not find
  function "xpath_element"</p>
</blockquote>

<p>The same happens if I load packages such as <code>httr</code>, <code>xml2</code>, <code>selectr</code>. I seem to have the latest version of these packages too... </p>

<p>In which packages are functions such as <code>xpath_element</code>, <code>xpath_combinedselector</code> located? How do I get it to work? Note that I am running on Ubuntu 16.04, so that code might work on other platforms...</p>