It was found useful in the ZOOM package to express the concept of a UnitVector, that is, a vector known to be inherently of unit length. CLHEP neither has such a class, and it is felt (at this time) that it should not. In the merged package, UnitVector is provided as a header file which appears only in the PhysicsVectors area.
Since UnitVector depends on no non-header implementation code, issues of where to place the library containing non-CLHEP code do not arise.
Although the UnitVector class is not derived from the SpaceVector class, all const methods (except for three relativistic kinematic methods which only make sense for a vector of length less than one) are provided for UnitVector. Thos non-const methods of Hep3Vector which do not risk violating the unit-length property, such as rotation, are also provided. In this section, therefore, only the differences in the classes are described.