CrossRatio Command

CrossRatio( <Point A>, <Point B>, <Point C>, <Point D> ): Calculates the cross ratio λ of four collinear points A, B, C and D, where: λ = AffineRatio[B, C, D] / AffineRatio[A, C, D].

CrossRatio((-1, 1), (1, 1), (3, 1), (4, 1)) yields 1.2