public function mustBeTypeOf(string $type) : Slot
    {
        static $allowed_types = [
            'boolean',
            'bool',
            'integer',
            'int',
            'float',
            'double',
            'string',
            'array',
            'object',
            'resource',
            'NULL',
            'null',
        ];
        $this->type = in_array($type,$allowed_types) ? $type : self::TYPE_ANY;
        return $this;
    }