title: String addition assignment in lisp
body: <p>I have a loop with a condition, based on which I decide whether I should append something to existing string or not.
  In Python, it should look like (this is dummy code, just to show the idea):</p>
  
  <pre><code>result_str = ''
  for item in range(5):
    if item % 2 == 0:
      result_str += str(item)
  
  print(result_str)
  </code></pre>
  
  <p>Output: <code>024</code></p>
  
  <p>So the question is: how can I perform addition assignment on strings (<code>+=</code>) in lisp?</p>