title: reduce with empty set as initial value
body: <p>I have a list of list and I want to construct a set out of the elements present in all the sublists..</p>
  
  <p>example :  <code>a = [[1,2],[2,3]]</code> should give <code>set([1,2,3])</code></p>
  
  <p>I tried reduce(lambda x,y:x.update(y),a,set([])) but it raises <code>AttributeError: 'NoneType'</code> object has no attribute 'update'</p>
  
  <p>Can someone tell me how to do this using reduce function ?</p>