code: func (l layouts) groupsSet() map[string]struct{} {
  	m := make(map[string]struct{})
  	for _, layout := range l {
  		for grp := range layout.QuantitiesByGroup {
  			m[grp] = struct{}{}
  		}
  	}
  	return m
  }
docs: // groupsSet returns a set of groups that the layouts contain