title: removing duplicate elements of a list with nested lists inside lisp
body: <p>The function remove-duplicates in lisp only works for simple lists like (a b c d).
  My question is that how I can remove duplicate elements from a list with nested lists inside it. So for example</p>
  
  <p>I have a list with nested lists as follows: ( (0 1) (0 2) (0 1) (0 3) )</p>
  
  <p>and I want to remove one of the (0 1) lists which is repeated twice inside the list above to get:</p>
  
  <p>( (0 1) (0 2) (0 3) )</p>