For example, if the character b were to follow tobeornottobe it would be encoded with probabilities (1/2,1/2, 3/26), without exclusion, leading to a coding requirement of 5.1 bits.
if exclusion was exploited, both encoder and decoder will recognize that escape from order 1 to order 0 is inevitable because the order 1 model adds no characters that were not already predicted by the order 2 model.