code: @JsonCreator
  	public static StringLinkRelation of(String relation) {
  
  		Assert.hasText(relation, "Relation must not be null or empty!");
  
  		return CACHE.computeIfAbsent(relation, StringLinkRelation::new);
  	}
docs: Returns a (potentially cached) {@link LinkRelation} for the given value.
  
  @param relation must not be {@literal null} or empty.
  @return