function ligatureSubstitutionFormat1(contextParams, subtable) {
    // COVERAGE LOOKUP //
    var glyphIndex = contextParams.current;
    var ligSetIndex = lookupCoverage(glyphIndex, subtable.coverage);
    if (ligSetIndex === -1) { return null; }
    // COMPONENTS LOOKUP
    // (!) note, components are ordered in the written direction.
    var ligature;
    var ligatureSet = subtable.ligatureSets[ligSetIndex];
    for (var s = 0; s < ligatureSet.length; s++) {
        ligature = ligatureSet[s];
        for (var l = 0; l < ligature.components.length; l++) {
            var lookaheadItem = contextParams.lookahead[l];
            var component = ligature.components[l];
            if (lookaheadItem !== component) { break; }
            if (l === ligature.components.length - 1) { return ligature; }
        }
    }
    return null;
}
Get feature indexes of a specific script
@param {string} scriptTag script tag