buildGroup = function(group, options) {
    if (!group) {
        return new mathMLTree.MathNode("mrow");
    }

    if (groupTypes[group.type]) {
        // Call the groupTypes function
        return groupTypes[group.type](group, options);
    } else {
        throw new ParseError(
            "Got group of unknown type: '" + group.type + "'");
    }
}
Takes a group from the parser and calls the appropriate groupTypes function
on it to produce a MathML node.