i></ul></p>
<p> </p>
</div><p><p>
  Returns the sorted iterator formed by merging <code>TupleIterator1</code> and
  <code>TupleIterator2</code>. The merge is performed on the <code>N</code>th element of
  each tuple. Both <code>TupleIterator1</code> and <code>TupleIterator2</code> must be
  key-sorted before evaluating this function. When two tuples compare
  equal, the tuple from <code>TupleIterator1</code> is picked before the tuple
  from <code>TupleIterator2</code>.</p>
 
  The first element of each ite