code: protected function createContext()
      {
          $indent        = isset($this->indent)        ? clone($this->indent) : new Indent();
          $renderer      = isset($this->renderer)      ? $this->renderer      : XmlRenderer::getInstance();
          $breakControl  = isset($this->breakControl)  ? $this->breakControl  : DefaultBreakControl::getInstance();
          return new DefaultContext($renderer, $indent, $breakControl);
      }
docs: この Builder にセットされている Indent, Renderer, BreakControl
  を使って新しい DefaultContext を生成します.
  
  @return DefaultContext 新しい DefaultContext