Function buildExpression

src/buildMathML.ts:168–238

(
    expression: AnyParseNode[],
    options: Options,
    isOrdgroup?: boolean,
)

 * <mtext> tag.
 */
export const buildExpression = function(
    expression: AnyParseNode[],
    options: Options,
    isOrdgroup?: boolean,
): MathNode[] {
    if (expression.length === 1) {
        const group = buildGroup(expression[0], options);
        if (isOrdgroup && group instanceof MathNode && group.type === "mo") {
            // When TeX writers want to suppress spacing on an operator,
            // they often put the operator by itself inside braces.
            group.setAttribute("lspace", "0em");
            group.setAttribute("rspace", "0em");
        }
        return [group];
    }

    const groups = [];
    let lastGroup;
    for (let i = 0; i < expression.length; i++) {
        const group = buildGroup(expression[i], options);
        if (group instanceof MathNode && lastGroup instanceof MathNode) {
            // Concatenate adjacent <mtext>s
            if (group.type === 'mtext' && lastGroup.type === 'mtext'
                && group.getAttribute('mathvariant') ===
                   lastGroup.getAttribute('mathvariant')) {
                lastGroup.children.push(...group.children);
                continue;
            // Concatenate adjacent <mn>s
            } else if (group.type === 'mn' && lastGroup.type === 'mn') {
                lastGroup.children.push(...group.children);
                continue;
            // Concatenate <mn>...</mn> followed by <mi>.</mi>
            } else if (isNumberPunctuation(group) && lastGroup.type === 'mn') {
                lastGroup.children.push(...group.children);
                continue;
            // Concatenate <mi>.</mi> followed by <mn>...</mn>
            } else if (group.type === 'mn' && isNumberPunctuation(lastGroup)) {
                group.children = [...lastGroup.children, ...group.children];
                groups.pop();
            // Put preceding <mn>...</mn> or <mi>.</mi> inside base of
            // <msup><mn>...base...</mn>...exponent...</msup> (or <msub>)
            } else if ((group.type === 'msup' || group.type === 'msub') &&
                group.children.length >= 1 &&
                (lastGroup.type === 'mn' || isNumberPunctuation(lastGroup))
            ) {
                const base = group.children[0];
                if (base instanceof MathNode && base.type === 'mn') {
                    base.children = [...lastGroup.children, ...base.children];
                    groups.pop();
                }
            // \not
            } else if (lastGroup.type === 'mi' && lastGroup.children.length === 1) {
                const lastChild = lastGroup.children[0];
                if (lastChild instanceof TextNode && lastChild.text === '\u0338' &&
                    (group.type === 'mo' || group.type === 'mi' ||
                        group.type === 'mn')) {
                    const child = group.children[0];
                    if (child instanceof TextNode && child.text.length > 0) {