Playfair's axiom is attractive for its economy of statement with few words. In 2011 Andy Liu of University of Alberta noted that "parallelism is transitive" forms an alternative, economic statement[10] The reformulation requires that the definition of parallel lines be that they do not have exactly one point in common. In this way the binary relation of parallelism on lines in the plane is reflexive. As Liu wrote, "Let P be a point not on line 2. Suppose both line 1 and line 3 pass through P and are parallel to line 2. By transitivity, they are parallel to each other, and hence cannot have exactly P in common. It follows that they are the same line, which is Playfair's axiom."