Class describing a 2D cartesian coordinate system
       (x, y coordinates)
       @ingroup GenVector

      Set internal data based on 2 Scalar numbers
      get internal data into 2 Scalar numbers
       set all values using cartesian coordinates
============= Compatibility section ================== The following make this coordinate system look enough like a CLHEP vector that an assignment member template can work with either
{ return X();}====== Set member functions for coordinates in other systems =======