public function users()
    {
        $wheres = [
            [$this->schema->objectClass(), Operator::$equals, $this->schema->objectClassUser()],
            [$this->schema->objectCategory(), Operator::$equals, $this->schema->objectCategoryPerson()]
        ];

        // OpenLDAP doesn't like specifying the omission of user objectclasses
        // equal to `contact`. We'll make sure we're working with
        // ActiveDirectory before adding this filter.
        if (is_a($this->schema, ActiveDirectory::class)) {
            $wheres[] = [$this->schema->objectClass(), Operator::$doesNotEqual, $this->schema->objectClassContact()];
        }

        return $this->where($wheres);
    }