const Distance& operator+=(const Distance &rhs) { value_+=meters_to_system(rhs.meters(),system_); return *this; }
const Distance& operator-=(const Distance &rhs) { value_-=meters_to_system(rhs.meters(),system_); return *this; }
const Distance& operator+=(const Distance &rhs) { value_+=meters_to_system(rhs.meters(),system_); return *this; }
const Distance& operator-=(const Distance &rhs) { value_-=meters_to_system(rhs.meters(),system_); return *this; }