WebCorpus with new feed items.corpus.update method ensures, that the original
WebCorpus feed sources are downloaded and checked against
already included TextDocuments. Based on the ID included
in the TextDocument's meta data, only new feed elements are
downloaded and added to the WebCorpus.
All relevant information regariding the original source feeds are stored
in the WebCorpus' meta data (meta).corpus.update(x, ...)WebCorpus