/**
 * Return a random alphanumerical string of length len
 * There is a very small probability (less than 1/1,000,000) for the length to be less than len
 * (il the base64 conversion yields too many pluses and slashes) but
 * that's not an issue here
 * The probability of a collision is tiny (need 3*10^12 documents to have one chance in a million of a collision)
 * See http://en.wikipedia.org/wiki/Birthday_problem
 */
export declare function uid(len: any): string;