title: OCaml directed graphs vertex module
body: <p>I have seen some graphs vertex signatures and even come up with my own:</p>
  
  <pre><code>module type VERTEX = sig
      type t
      type label
  
      val equal : t -&gt; t -&gt; bool
      val create : label -&gt; t
      val label : t -&gt; label  
  end
  </code></pre>
  
  <p>But I have completely no idea how to implement it as a module. What types should t and label be? How can I create a t based on a label? And how do I get the label from a t?</p>