title: How this operation is called?
body: <p>I'm trying to solve a problem that I believe is a standard Python task and has already been asked here, but I don't know the proper vocabulary term for it.</p>
  
  <p>Given the structure</p>
  
  <pre><code>((['a'], t1), (['a', 'b', 'c'], t2), (['c', 'd'], t3))
  </code></pre>
  
  <p>and I need to get something like this</p>
  
  <pre><code>(('a', [t1, t2]), ('b', [t2]), ('c', [t2, t3]), ('d',[t3]))
  </code></pre>
  
  <p>out of given data. How to solve this task or at least what is the name of this operation?</p>