code: @Override
    public StreamBuilderImpl<T,U> reduce(T init,
                                         BinaryOperatorSync<T> op)
    {
      return new ReduceOpInitSync<>(this, init, op);
    }
docs: Reduce with a binary function
  
  <code><pre>
  s = init;
  s = op(s, t1);
  s = op(s, t2);
  result = s;
  </pre></code>