title: store a text string in latex and then add other text to it (concatenate)
body: <p>I start by defining a command to store the string "Hello":</p>
  
  <pre><code>\newcommand{\textstring}{Hello}
  </code></pre>
  
  <p>I would like to append the string " world" but unfortunately this code causes an error:</p>
  
  <pre><code>\renewcommand{\textstring}{\textstring world}
  </code></pre>