code: public Relation<K,V> create(Relation<K,V> r) {
  	// default implementation
  	Relation<K,V> r2 = this.create();
  	for(K key : r.keys()) {
  	    r2.addAll(key, r.getValues(key));
  	}
  	return r2;
      }
docs: Default implementation: uses {@link #create()} to create an
  empty relation, and next add each pair from <code>r</code> to
  the new relation.