'Look backwards k tokens on-channel tokens'

def LB(self, k):

    if (self.p == (-1)):
        self.fillBuffer()
    if (k == 0):
        return None
    if ((self.p - k) < 0):
        return None
    i = self.p
    n = 1
    while (n <= k):
        i = self.skipOffTokenChannelsReverse((i - 1))
        n += 1
    if (i < 0):
        return None
    return self.tokens[i]
