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Context: We then experiment with injecting the patterns back into the pre-trained transformer encoder.In particular, we injectthem through additional attention heads in the form of a Projected Attention Layer (PAL) (Stickland and Murray, 2019), along with the parameters of the original model. Details regarding PALs are described in Appendix A.

Question: How do Projected Attention Layers work?
Projected Attention Layers (PAL) takes the hidden state from the previous layer and runs parallel to the self-attention layer. In each PAL, the hidden size of the pretrained layer is linearly projected and then passed through its own self-attention layer before undergoing transformation back to the original hidden state size.