An arrow in \(\RNrSpc{n}\) is given by listing two points \(P\) and \(Q\) of \(\RNrSpc{n}\) in order. The first point \(P\) is called the tail of the arrow. The second point \(Q\) is called the tip of the arrow. We write \(\Arrow{P}{Q}\) to denote the arrow joining \(P\) to \(Q\).