title: Remove arrays from nested arrays based on first element of each array
body: <p>I have two nested arrays say </p>
  
  <pre><code>a=[[1,2,3],[2,4,7],[4,2,8],[3,5,7],[6,1,2]]
  b=[[1,6,7],[2,4,9],[4,3,5],[3,10,2],[5,3,2],[7,2,1]]
  </code></pre>
  
  <p>I want to only keep those arrays in <code>b</code> whose first element is not common to the first elements of the arrays in <code>a</code>, so for these two we should get</p>
  
  <pre><code>c=[[5,3,2],[7,2,1]]
  </code></pre>
  
  <p>Is there a way to do this in python?</p>