mathDefault = function(value, mode, color, classes, type) {
    if (type === "mathord") {
        return mathit(value, mode, color, classes);
    } else if (type === "textord") {
        return makeSymbol(
            value, "Main-Regular", mode, color, classes.concat(["mathrm"]));
    } else {
        throw new Error("unexpected type: " + type + " in mathDefault");
    }
}
Makes a symbol in the default font for mathords and textords.