code: public Link getRequiredLink(LinkRelation relation) {
  
  		Assert.notNull(relation, "Link relation must not be null!");
  
  		return getRequiredLink(relation.value());
  	}
docs: Returns the link with the given relation.
  
  @param relation must not be {@literal null}.
  @return the link with the given relation.
  @throws IllegalArgumentException in case no link with the given relation can be found.