title: return a list of element from a list in OCaml
body: <p>I am new to OCaml, and I am now trying to implement a function that returns a list of elements of a given list <code>x</code> at indexes in list <code>y</code>.</p>
  
  <p>For example, that function should perform the following computation: <code>[5,6,7,8], [0, 3] =&gt; [5, 8]</code></p>
  
  <p>I am not sure how to store temp variables in ML and don't have a clear idea how it works. I do know how to find a element from a list given specified index, though.</p>
  
  <p>Any idea will be appreciated, but I'd like to use recursive functions and avoid the <code>List</code> module.</p>