title: Variadic curried sum function
body: <p>I need a js sum function to work like this:</p>
  
  <pre><code>sum(1)(2) = 3
  sum(1)(2)(3) = 6
  sum(1)(2)(3)(4) = 10 
  etc.
  </code></pre>
  
  <p>I heard it can't be done. But heard that if adding <code>+</code> in front of <code>sum</code> can be done.
  Like <code>+sum(1)(2)(3)(4)</code>. <br>Any ideas of how to do this?</p>