src/main/java/com/theoryinpractise/halbuilder5/ResourceRepresentation.java
public ResourceRepresentation<V> withRel(Rel rel) {
    if (rels.containsKey(rel.rel())) {
      throw new IllegalStateException(String.format("Rel %s is already declared.", rel.rel()));
    }
    final TreeMap<String, Rel> updatedRels = rels.put(rel.rel(), rel);
    return new ResourceRepresentation<>(
        content, links, updatedRels, namespaceManager, value, resources);
  }
public ResourceRepresentation<V> withRel(Rel rel) {
    if (rels.containsKey(rel.rel())) {
      throw new IllegalStateException(String.format("Rel %s is already declared.", rel.rel()));
    }
    final TreeMap<String, Rel> updatedRels = rels.put(rel.rel(), rel);
    return new ResourceRepresentation<>(
        content, links, updatedRels, namespaceManager, value, resources);
  }
Define rel semantics for this representation.

@param rel A defined relationship type