public Double exist(LinkRelevance linkRelev) {
        LinkRelevance link = urlRelevance.get(linkRelev.getURL().toString());
        return link == null ? null : link.getRelevance();
    }
It verifies whether a given URL was already visited or does not belong to
the scope.

@param linkRelev
@return
ache/src/main/java/achecrawler/link/frontier/Frontier.java