title: uniq elements from array of arrays
body: <p>I want to find unique elements from an array of arrays by the first element in the inner arrays.</p>
  
  <p>for example</p>
  
  <pre><code>a = [[1,2],[2,3],[1,5]
  </code></pre>
  
  <p>I want something like</p>
  
  <pre><code>[[1,2],[2,3]]
  </code></pre>