Second, arc AB= arc CD means that two arcs can completely overlap.
It is only the same length or degree, so it is not allowed.
Because arcs have lengths and radians, arcs with equal lengths are not necessarily equal, and arcs with equal radians are not necessarily equal in length.
Third, you can write the formula of arc AB +/- arc CD (in the same circle or equal circle).
(that is, a usage of the sum line segment)