mathrm = function(value, mode, color, classes) {
    // Decide what font to render the symbol in by its entry in the symbols
    // table.
    if (symbols[mode][value].font === "main") {
        return makeSymbol(value, "Main-Regular", mode, color, classes);
    } else {
        return makeSymbol(
            value, "AMS-Regular", mode, color, classes.concat(["amsrm"]));
    }
}
Makes a symbol in the upright roman font.