code: public Set<EnumType> getEnumTypes() {
          Set<EnumType> ret = Sets.newTreeSet();
          for (Entity entity : getEntities().getList()) {
              for (Attribute attribute : entity.getAttributes().getList()) {
                  if (attribute.isEnum()) {
                      ret.add(attribute.getEnumType());
                  }
              }
          }
          return ret;
      }
docs: TODO: we should have a central place for enum types