title: How to take product of two list in OCaml?
body: <p>I have two lists :</p>
  
  <pre><code>let a = ["a";"b"];
  let b = ["c";"d"];
  </code></pre>
  
  <p>I want an output list c such as :</p>
  
  <pre><code>c = ["a";"c";"a";"d";"b";"c";"b";"d"];
  </code></pre>
  
  <p>How to do it in ocaml as lists are immutable? I am new to it.</p>