public IEnumerable <DocumentId> GetChangedAdditionalDocuments()
{
// if the document states are different then there is a change.
foreach (var id in _newProject.AdditionalDocumentIds)
{
var newState = _newProject.GetAdditionalDocumentState(id);
var oldState = _oldProject.GetAdditionalDocumentState(id);
if (oldState != null && newState != oldState)
{
yield return(id);
}
}
}