In the front matter, the boldface 1 is defined as the indicator function, which takes values of one or zero depending on whether the condition in the subscript is satisfied. However, in equation 10.18,

the boldface 1 does not have a condition in its subscript (i, y^(t) is not a condition). This is a typo. The subscript of the boldface 1 should be i = y^(t).

This section is also confusing because the authors state that the loss of the model is the negative log-likelihood of the "true target y^(t) given the input so far." The loss is the negative log of the model probability for the true label y^(t), not the negative log of the true probability.

Following the above assumptions, one can derive the corrected version of equation 10.18.