void check_fuel(String chain){
    boolean fuelExists = find_x == string::npo;
	if (fuelExists) {
		for (int i = 0; i < chain.size(); i++) {
			if (chain[i] == 'P' || chain[i] == 'T' || chain[i] == 'X' || chain[i] == 'N')
				continue;
			else
				return f;
		}
	}
	else {
		for (int i = 0; i < chain.size(); i++){
			if (chain[i] == 'P' || chain[i] == 'T' || chain[i] == 'N')
				continue;
			else
				return f;
	}
}