title: Merging 2 dictionaries and saving translation dictionary for the overridden values
body: <p>I merge 2 dictionaries, say <code>d1</code> and <code>d2</code> by:</p>
  
  <pre><code>d1.update(d2)
  </code></pre>
  
  <p>i.e I have:</p>
  
  <pre><code>d1 = {1:'a', 2:'b', 3:'c'}
  d2 = {1:'a', 2:'g', 4:'x'}
  </code></pre>
  
  <p>then after <code>d1.update(d2)</code> I have:</p>
  
  <pre><code>{1: 'a', 2: 'g', 3: 'c', 4: 'x'}
  </code></pre>
  
  <p>So the original <code>2:'b'</code> vanished so I'd like to have the translation 'b':'g' somehow, meaning the output will be the following dictionary:</p>
  
  <pre><code>{'b':'g'}
  </code></pre>
  
  <p>Is there any simple code to keep the overridden values?</p>