title: Idris - derive extended interface instance
body: <p>Suppose I have a function <code>f : Ord a =&gt; ...</code> which requires <code>a</code> to have and <code>Ord</code> instance.
  I can access the <code>Ord a</code> instance using</p>
  
  <pre><code>f : Ord a =&gt; ...
  f @{ord} ...
  </code></pre>
  
  <p>Since <code>Eq a =&gt; Ord a</code>, <code>a</code> needs to have also an <code>Eq a</code> instance. Is there a way to retrieve it directly from <code>Ord a</code>, instead of doing something like the following?</p>
  
  <pre><code>f : (Eq a, Ord a) =&gt; ...
  f @{eq} @{ord} ...
  </code></pre>