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On the page 220, in the section 12.16 backpropagation with arbitrary length, there is a mistmatch between book description of the code and code, which explains the section, namely:

The most important object is our layers list, which has two vectors (layer['state'] and
layer['previous->hidden']). In order to backpropagate, we're going to take our output
gradient and add a new object to each list called layer['state_delta'] which is going to
represent the gradient at that layer.

First of all, there are no layer list in the code

Secondly, the variables in the code on the same page are being named as:

layer['hidden'] instead of layer['state'], and layer['hidden_delta'] instead of layer['state_delta']

I'm not able to find, to which variable the layer['previous->hidden'] refers to.