docs: public void setReference(final String propertyName, final BeanId value) {
          Preconditions.checkNotNull(propertyName);
          if (value == null) {
              references.put(propertyName, null);
              return;
          }
          checkCircularReference(value);
          List<BeanId> values = new ArrayList<>();
          values.add(value);
          references.put(propertyName, values);
      }