title: Recursive function to repeat string in OCaml
body: <p>I am absolute OCaml beginner. I want to create a function that repeats characters 20 times.
  This is the function, but it does not work because of an error.</p>
  
  <pre><code>let string20 s =
    let n = 20 in
    s ^ string20 s (n - 1);;
  
  string20 "u";;
  </code></pre>
  
  <p>I want to run like this</p>
  
  <pre><code># string20 "u"
  - : string = "uuuuuuuuuuuuuuuuuuuu"
  </code></pre>