code: func (ct *CombinedTags) AddTags(concat bool, tagsList ...Tagged) {
  	for _, t := range tagsList {
  		for _, k := range t.Keys() {
  			if len(ct.ks[k]) == 0 {
  				ct.keys = append(ct.keys, k)
  				// Store reference to key-holder amongst Taggeds
  				ct.ks[k] = append(ct.ks[k], t)
  			} else if concat {
  				// Store additional tag reference only if concat, otherwise first key-holder wins
  				ct.ks[k] = append(ct.ks[k], t)
  			}
  		}
  	}
  }
docs: // Adds each of tagsList to CombinedTags - choosing whether values for the same key should
  // be concatenated or whether the first should value should stick