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