code: @Override
    public StreamBuilderImpl<T,U> reduce(BinaryOperatorSync<T> op)
    {
      return new ReduceOpSync<>(this, op);
    }
docs: reduce((x,y)->op) with a sync binary function
  
  <code><pre>
  s = t1;
  s = op(s, t2);
  s = op(s, t3);
  result = s;
  </pre></code>