code: static void validateExtends(EntityType entityType) {
      EntityType entityTypeExtends = entityType.getExtends();
      if (entityTypeExtends != null && !entityTypeExtends.isAbstract()) {
        throw new MolgenisValidationException(
            new ConstraintViolation(
                format(
                    "EntityType [%s] is not abstract; EntityType [%s] can't extend it",
                    entityTypeExtends.getId(), entityType.getId())));
      }
    }
docs: Validates if this entityType extends another entityType. If so, checks whether that parent
  entityType is abstract.
  
  @param entityType entity meta data
  @throws MolgenisValidationException if the entity extends from a non-abstract entity