title: How to append items in dictionary nested list?
body: <p>I have a file with two columns of data, for example:</p>
  <pre><code>1  5
  1  6
  2  3
  3  4
  3  5
  ...
  </code></pre>
  <p>I want to obtain a dictionary with first column as the key and the second column as values, something like the following:</p>
  <pre><code>B = {1 : [5,6], 2: [3], 3: [4,5]}
  </code></pre>