LB method
Implementation
@override
Token? LB(int k) {
if (k == 0 || (p - k) < 0) return null;
var i = p;
var n = 1;
// find k good tokens looking backwards
while (n <= k && i > 0) {
// skip off-channel tokens
i = previousTokenOnChannel(i - 1, channel);
n++;
}
if (i < 0) return null;
return tokens[i];
}