How to use the speech-rule-engine.toSpeech function in speech-rule-engine

if (!result.mml) result.mml = result.mmlNode.outerHTML;
    if (!speechConfig.speech) speechConfig.speech =  'deep';
    // add semantic tree
    if (speechConfig.semantic) {
        result.streeJson = sre.toJson(result.mml);
        const xml = sre.toSemantic(result.mml).toString();
        result.streeXml = speechConfig.minSTree ? xml : sre.pprintXML(xml);
    }
    // return if no speakText is requested
    if (!speechConfig.speakText) {
        callback(result, input);
        return
    }
    // enrich output
    sre.setupEngine(speechConfig);
    result.speakText = sre.toSpeech(result.mml);
    if (result.svgNode) {
        result.svgNode.querySelector('title').innerHTML = result.speakText;
        // update serialization
        // HACK add lost xlink namespaces TODO file jsdom bug
        if (result.svg) result.svg = result.svgNode.outerHTML
            .replace(/><([^/])/g, ">\n<$1")
            .replace(/(<\/[a-z]*>)(?=<\/)/g, "$1\n")
            .replace(/(<(?:use|image) [^>]*)(href=)/g, ' $1xlink:$2');
    }
    if (result.htmlNode) {
        result.htmlNode.firstChild.setAttribute("aria-label", result.speakText);
        // update serialization
        if (result.html) result.html = result.htmlNode.outerHTML;
    }
    if (result.mmlNode) {
        result.mmlNode.setAttribute("alttext", result.speakText);

const preprocessor = function (data, callback) {
    // setup SRE
    const speechConfig = {
        semantics: true,
        domain: data.speakRuleset || 'mathspeak',
        style: data.speakStyle || 'default',
        semantic: data.semantic,
        minSTree: data.minSTree,
        speakText: true,
        speech: 'deep'
    };
    if (data.speakText === false) speechConfig.speakText = false
    sre.setupEngine(speechConfig);
    // if MathML, enrich and continue
    if (data.format === "MathML") {
        data.speakText = sre.toSpeech(data.math);
        data.math = sre.toEnriched(data.math).toString();
        data.math = data.math.replace(/alttext="(.*?)"/,'alttext="' + data.speakText + '"');
        callback(data);
    } else {
        // convert to MathML, enrich and continue
        const newdata = {
            math: data.math,
            format: data.format,
            mml: true
        };
        mathjax.typeset(newdata, function (result) {
            if (result.error) throw result.error;
            data.speakText = sre.toSpeech(result.mml);
            data.math = sre.toEnriched(result.mml).toString();
            data.math = data.math.replace(/alttext="(.*?)"/,'alttext="' + data.speakText + '"');
            data.format = 'MathML';

mathjax.typeset(newdata, function (result) {
            if (result.error) throw result.error;
            data.speakText = sre.toSpeech(result.mml);
            data.math = sre.toEnriched(result.mml).toString();
            data.math = data.math.replace(/alttext="(.*?)"/,'alttext="' + data.speakText + '"');
            data.format = 'MathML';
            callback(data);
        });
    }