horizontalPoint and verticalPoint must both be instances of the same domain.
RelativeEndPoints establishes the endpoints of two vectors which describe the horizontal and vertical orientation of the text. Both vectors start at the origin of the text string, assumed to be (0,0). In the CGM (ANSI) specification, the vectors exist in the (x,y) plane. However, through subclasses of the Coordinate subclass, as used by Point, also permits the definition of z and time.