title: map-invert with a tweak for a map whose values are sequences
body: <p>How to invert a map whose values are sequences?
  In the inverted map the keys are the entries of all the sequences (assuming no duplicate keys)</p>
  
  <p>For instance:</p>
  
  <pre><code>(map-invert-tweaked {:monday [:banana :apple]
                       :sunday [:pineapple :mango]}); {:banana :monday
                                                       :apple  :monday
                                                       :pineapple :sunday
                                                       :mango     :sunday}
  </code></pre>
  
  <p>I hope that the question is well-defined.</p>