Increase or reset sequence counter and recalculate prefix when needed.

@return {string}
function getSequence() {
    var mod = ++seq % 16

    // reset counter
    if (seq > 0xffffff)
        seq = mod = 0

    // If counter is divisible by 16 then
    // the sequence prefix should be regenerated.
    // Otherwise only the last digit changed,
    // so we don't need to update the prefix.
    if (!mod)
        seqpref = pad(~~(seq / 16), 5, 16)

    return seqpref + digits[ mod ]
}