sentences:  For example, an equation of "7.6 back = 9.2 ahead" means that the feature does not have any section between mile 7.6 and mile 9.2, and the distance between mileposts 7 and 10 is only 1.4 miles.This would usually be caused by a relocation that shortened the distance by 1.6 miles.It is also possible for an equation to add mileage to what it would otherwise be; the duplicated mileposts receive a special prefix, such as Z.