code: private void validateSubEntities(Entity obj) {
          // Validate that all sub entities have a "rel" set.
          String relRequired = "Sub entities are required to have a <rel> property set.";
          String hrefReqForEmbed = "Sub entities that are embedded links are required to have a <href> property set.";
          if (!CollectionUtils.isEmpty(obj.getEntities())) {
              for (Entity e : obj.getEntities()) {
                  if (e.getRel() == null || ArrayUtils.isEmpty(e.getRel())) {
                      throw new Siren4JBuilderValidationException("entities", obj.getClass(), relRequired);
                  }
                  if ((e instanceof EmbeddedEntityImpl) && StringUtils.isBlank(e.getHref())) {
                      throw new Siren4JBuilderValidationException("entities", obj.getClass(), hrefReqForEmbed);
                  }
              }
          }
      }
docs: Validate sub entities ensuring the existence of a relationship and if an embedded link then ensure the
  existence of an href.
  @param obj assumed not <code>null</code>.