buildExpression = function(expression, options) {
    var groups = [];
    for (var i = 0; i < expression.length; i++) {
        var group = expression[i];
        groups.push(buildGroup(group, options));
    }
    return groups;
}
Takes a list of nodes, builds them, and returns a list of the generated
MathML nodes. A little simpler than the HTML version because we don't do any
previous-node handling.