code: public void addReference(final String propertyName, final Collection<BeanId> refs) {
          Preconditions.checkNotNull(refs);
          Preconditions.checkNotNull(propertyName);
          checkCircularReference(refs.toArray(new BeanId[refs.size()]));
          List<BeanId> list = references.get(propertyName);
          if (list == null) {
              list = new ArrayList<>();
              list.addAll(refs);
              references.put(propertyName, list);
          } else {
              list.addAll(refs);
          }
      }
docs: Add a list of references to a property on this bean.
  
  A reference identify other beans based on schema and instance id.
  
  @param propertyName name of the property as defined by the bean's schema.
  @param refs the reference as defined by the bean's schema.