let parse = function (expr) {
    let regex = /\S+/g;
    let tokens = [];
    let s;
    while (s = regex.exec(expr)) {
        tokens.push(s[0]);
    }
    function expression() {
        let token = tokens.pop();
        if (!isNaN(token)) {
            return new Const(Number(token));
        }
        if (indexes.has(token)) {
            return new Variable(token);
        }
        let args = [];
        for (let i = 0; i < operations[token].argsCount; i++) {
            args.push(expression());
        }
        args.reverse();
        return createOperation(operations[token].Constructor, args);
    }
    return expression();
};