lass="special">,</span> <span class="keyword">typename</span> BinaryPredicate<span class="special">&gt;</span> 
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span> <span class="identifier">OutputKeyIterator</span><span class="special">,</span> <span class="identifier">OutputValueIterator</span> <span class="special">&gt;</span> 
  <span class="identifier">reduce_by_key</span><span class="special">(</span><span class="ident