operator/(const Coord& i)Create a new group formed by restricting this group by i. The inline definition is
Group operator/(const Coord& i) const { Group result(*this) ; result.restrict(i.dim, i.crd) ; return result ; }
operator/=(const Coord& i)Restrict this group by i. The inline definition is
inline Group& operator/=(const Coord& i) { restrict(i.dim, i.crd) ; return *this ; }