code: public static Collection<String> getAllTagNames(Collection<Channel> channels) {
  		Collection<String> tagNames = new HashSet<String>();
  		for (Channel channel : channels) {
  			tagNames.addAll(getTagNames(channel));
  		}
  		return tagNames;
  	}
docs: Return a union of tag names associated with channels
  
  @param channels - list of channels
  @return a set of all unique tag names associated with atleast one or more
  channel in channels