title: Brace expansion as flag argument pairs
body: <p>I'm trying to use <code>top</code> in conjunction with <code>pgrep</code> to filter the processes that are being shown. What I have working so far is:</p>
  <pre class="lang-sh prettyprint-override"><code>top -pid (pgrep dart)
  </code></pre>
  <p>Which works great for getting a single process to show in <code>top</code>'s interactive view.</p>
  <p>However, the macos version of <code>top</code> only has one way of listing multiple processes, you have to just repeat <code>-pid [process id]</code> again and again eg:</p>
  <pre class="lang-sh prettyprint-override"><code>top -pid 354 -pid 236 -pid 287
  </code></pre>
  <p>My first thought was that I could use brace expansion and command substitution to achieve this, and I tried:</p>
  <pre class="lang-sh prettyprint-override"><code>top &quot;-pid &quot;{(pgrep dart)}
  </code></pre>
  <p>But I get <code>invalid option or syntax: -pid {33978}</code>. Even when I manually add in pids it doesn't work:</p>
  <pre><code>top &quot;-pid &quot;{45, 23}
  
  invalid option or syntax: -pid 45
  </code></pre>
  <p>Is it possible to achieve what I'm trying to do with fish? i.e insert flags into a command via a combination of command substitution and brace expansion?</p>