docs: private ImmutableSet<Type> targetTypes(Computable.Targetable<?> query) {
          if (query.of().isEmpty()) {
              throw GraqlQueryException.statisticsAttributeTypesNotSpecified();
          }
  
          return query.of().stream()
                  .map(t -> {
                      Label label = Label.of(t);
                      Type type = tx.getSchemaConcept(label);
                      if (type == null) throw GraqlQueryException.labelNotFound(label);
                      if (!type.isAttributeType()) throw GraqlQueryException.mustBeAttributeType(type.label());
                      return type;
                  })
                  .flatMap(Type::subs)
                  .collect(CommonUtil.toImmutableSet());
      }