title: "as" keyword in OCaml
body: <p>In the answers for the tutorials for OCaml available at <a href="http://ocaml.org/learn/tutorials/99problems.html" rel="noreferrer">this site</a>, some of the solutions, including the one for eliminating consecutive duplicates of list elements, is written as such:</p>
  
  <pre><code>let rec compress = function
      | a :: (b :: _ as t) -&gt; if a = b then compress t else a :: compress t
      | smaller -&gt; smaller;;
  </code></pre>
  
  <p>What is the relevance of the line <code>a :: (b:: _ as t)</code>? Why can't I write it as <code>a :: b :: t</code> instead?</p>