How to use the genie-toolkit.Contextualizer function in genie-toolkit

.pipe(new Genie.DatasetStringifier())
                .pipe(fs.createWriteStream(basicDataset, { fd: basicDatasetFD })));
            // basicDatasetFD is closed here

            let contexts = await
                fs.createReadStream(basicDataset, { encoding: 'utf8' })
                .pipe(byline())
                .pipe(new Genie.DatasetParser({ contextual: false }))
                .pipe(new Genie.ContextExtractor(this._schemas))
                .read();

            const contextualized =
                fs.createReadStream(basicDataset, { encoding: 'utf8' })
                .pipe(byline())
                .pipe(new Genie.DatasetParser({ contextual: false }))
                .pipe(new Genie.Contextualizer(contexts, {
                    locale: this._language,
                    numSamples: 20,
                    nullOnly: false,
                }));

            const contextualSynthetic = genSynthetic.generate(tmpDir, {
                contextual: true,
                contexts,

                language: this._language,
                flags: this._options.flags,
                maxDepth: this._options.maxDepth,
                targetPruningSize: this._options.contextualTargetPruningSize,
                debug: this._options.debug,
            });