Signatures of Hep3Vector::rotate For equivalent ZOOM axis There is no harm in leaving this axis CLHEP has implemented a first forming an identity then rotating that by axis and I leave the CLHEP code alone people are of course free to use the ZOOM originated method with signature which I believe will be faster Return types for rotateZ CLHEP and PhysicsVectors each have these three and they are identical except that the ZOOM version returns a reference to while in CLHEP they return void Having methods that alter an object return a reference to that object is convenient for certain chained and costs nothing I don t wish to potentially break ZOOM user code for no good so I have made these CLHEP method conform to this convention There are a couple of other CLHEP rotate and which use the void return type