avoidNumber = str => {
	// max length of a number is 21 chars, bigger numbers a written as "...e+xx"
	if (str.length > 21) return str;
	const firstChar = str.charCodeAt(0);
	// skip everything that doesn't look like a number
	// charCodes: "-": 45, "1": 49, "9": 57
	if (firstChar < 49) {
		if (firstChar !== 45) return str;
	} else if (firstChar > 57) {
		return str;
	}
	if (str === String(Number(str))) {
		return `_${str}`;
	}
	return str;
}
@param {string} str the string
@returns {string} string prefixed by an underscore if it is a number