code: public Relation<V,K> revert(final Relation<V,K> result) {
  	forAllEntries(new EntryVisitor<K,V>() {
  		public void visit(K key, V value) {
  		    result.add(value, key);
  		}
  	    });
  	return result;
      }
docs: Revert <code>this</code> relation and store the result into
  the relation <code>result</code>. &lt;a,b&gt; appears in the
  reverse relation iff &lt;b,a&gt; appears in <code>this</code>
  relation.  Returns the new relation (ie,
  <code>result</code>).