IQueryable<ArticleFragment> fragments = from f in this.Catalog.ArticleFragments
                                        select f;
foreach (string term in query.Include)
{
    fragments = fragments.Where(f => f.Fragment.Contains(term));
}

foreach (string term in query.Exclude)
{
    fragments = fragments.Where(f => !f.Fragment.Contains(term));
}

public INavigable<SearchResult<Article>> Search(SearchQuery query)
{
    IQueryable<SearchResult<Article>> q =
        from f in this.Catalog.ArticleFragments
        where query.Include.Any(i => f.Fragment.Contains(i)) &&
                query.Exclude.All(i => !f.Fragment.Contains(i))
        group f by f.Article.Id into g
        let weight = g.Max(w => w.Weight)
        join a in this.Catalog.Articles on g.Key equals a.Id
        orderby weight descending
        select new SearchResult<Article>
        {
            Result = a,
            Relevance = weight
        };

    return new QueryNavigator<SearchResult<Article>>(q);
}