code: public Link getRequiredLink(String relation) {
  
  		return getLink(relation) //
  				.orElseThrow(() -> new IllegalArgumentException(String.format("No link with rel %s found!", relation)));
  	}
docs: Returns the link with the given relation.
  
  @param relation must not be {@literal null} or empty.
  @return the link with the given relation.
  @throws IllegalArgumentException in case no link with the given relation can be found.