fier">OutputValueIterator</span> <span class="special">&gt;</span> 
  <span class="identifier">reduce_by_key</span><span class="special">(</span><span class="identifier">InputKeyIterator</span> keys_first<span class="special">,</span> <span class="identifier">InputKeyIterator</span> keys_last<span class="special">,</span> 
                <span class="identifier">InputValueIterator</span> values_first<span class="special">,</span> 
                <span class="identifier">OutputKeyIterator</span> keys_resul