title: Sum of even-numbered items in Seq
body: <p>I'm trying to sum up even numbered items, and I went for making tuples and summing the first values in them, but It's creating a lot of objects which are immediately discarded.</p>
  
  <p>Maybe someone can suggest more lightweight solution (maybe with <code>fold</code>)?</p>
  
  <p>Edit: I meant numbers on even-numbered <strong>positions</strong></p>
  
  <p>Testcase:</p>
  
  <pre><code>List(1,3,1,3).foldLeft(x)(magic) === 2
  </code></pre>