public void visitTuples(Visitor<Tuple<T>> visitor) {
        try (TupleIterator<T> it = iterator()) {
            while (it.hasNext()) {
                visitor.visit(it.next());
            }
        } catch (Exception e) {
            logger.error("Failed to close iterator.", e);
        }
    }
DEPRECATED: may cause OutOfMemoryError on large crawls.

@return
ache/src/main/java/achecrawler/util/persistence/PersistentHashtable.java