code: private static List<DAType> computeExtendedInterfaces(List<DAInterface> interfaces) {
      Optional<DAType> functionInterface = from(interfaces)
          .filter(DAInterfacePredicates.isGuavaFunction())
          .transform(toDAType())
          .filter(notNull())
          .first();
      if (functionInterface.isPresent()) {
        return Collections.singletonList(functionInterface.get());
      }
      return  Collections.emptyList();
    }
docs: The only interface that can be extended by the Mapper interface is Guava's Function interface.