private XDocument LoadPart(OpenXmlPart source)
{
if (source == null) return null;
var part = source.Annotation<XDocument>();
if (part != null) return part;
using (var str = source.GetStream())
using (var streamReader = new StreamReader(str))
using (var xr = XmlReader.Create(streamReader))
part = XDocument.Load(xr);
return part;
}