that a rule uses to build with. They can refer either
    to other rules within the repo, or system-level binaries, for example:
  </p>

  <pre class="code-container">
    <!-- prettier-ignore -->
    <code data-lang="plz">
    tools = [
        'curl',
        '//path/to:tool',
    ],
    </code>
  </pre>

  <p>
    In this example, <code class="code">curl</code> is resolved on the system
    using the <code class="code">PATH</code> variable defined in your
    <a class="copy-link" href="/config.html#build