LB() can try to scan to i < 0

[parrt]

LB scans w/o checking i:

        while ( n<=k ) {
            // skip off-channel tokens
            i = previousTokenOnChannel(i - 1, channel);
            n++;
        }

then we fail an assertion in sync() called from previousTokenOnChannel():

    protected boolean sync(int i) {
        assert i >= 0;
        int n = i - tokens.size() + 1; // how many more elements we need?
...
    }