User.prototype.inc = function(field, value) {
    this[field] += value;
    return this.save();
}

User.prototype.dec = function(field, value) {
    this[field] -= value;
    return this.save();
}

User.prototype.set = function(field, value) {
    this[field] = value;
    return this.save();
}
function flipString(string) {
    let result = "";
    string = string.toLowerCase().split("").reverse();
    string.map((symbol) => {
        result += flipTable[symbol] || symbol;
    });
    return result;
}