title: assign value at specific list index in robot framework
body: <p>I have a list:</p>
  
  <pre><code>@{IFUP}  10  20
  </code></pre>
  
  <p>I want to modify only one of those values, e.g:</p>
  
  <pre><code>${IFUP}[${idx}]=  Set Variable  30
  </code></pre>
  
  <p>where $idx is 0</p>
  
  <p>This produces <code>No keyword with name ''${IFUP}[${idx}]='</code>. Same happens with direct <code>${IFUP}[0]</code> assignment. </p>
  
  <p>RobotFramwork version is 3.1 (list syntax is a bit different).</p>
  
  <p>I would be happy with a variable variable solution like: <code>${IFUP_${idx}}=</code> but this produces the same error.</p>
  
  <p>Suggestions?</p>