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Adapted from ref (12). (A) is the starting S2 state, (D) is the final S3 state, and (B) and (C) are proposed intermediate steps for the S2 to S3 state transition. W1, W2, and W3 correspond to W2, W1, and Wx in ref (12), respectively, while O5 has been relabeled as O6 following ref (13), after W1 moves into the O5 position to become O5.
