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];
    }
Takes a list of nodes, builds them, and returns a list of the generated
MathML nodes.  Also combine consecutive <mtext> outputs into a single
<mtext> tag.