title: algorithm to sum up a list of numbers for all combinations
body: <p>I have a list of numbers and I want to add up all the different combinations.
  For example:</p>
  
  <ul>
  <li>number as 1,4,7 and 13</li>
  <li>the output would be:</li>
  </ul>
  
  <p><br></p>
  
  <pre><code>1+4=5
  1+7=8
  1+13=14
  4+7=11
  4+13=17
  7+13=20
  1+4+7=12
  1+4+13=18
  1+7+13=21
  4+7+13=24
  1+4+7+13=25
  </code></pre>
  
  <p>Is there a formula to calculate this with different numbers?</p>