private void AddEntry(string word, int occurenceCount, Document document)
        {
            if (!index.ContainsKey(word))
            {
                index[word] = new PostingsList();
            }

            index[word].AddDocument(document.ID, occurenceCount);
        }
        public void NewDocument(Uri uri, string content)
        {
            Document doc = new Document(nextID, uri, content);

            documentsByURI.Add(uri, doc);
            documentsByID.Add(nextID, doc);

            nextID++;

            foreach (IGrouping<string, string> word in doc.Words.GroupBy(w => w))
            {
                AddEntry(word.Key, word.Count(), doc);
            }
        }