A user prompts a language model to write a short summary of a provided article. The model generates a one-paragraph summary. The user then provides feedback: 'This is too brief. Please expand on the key findings mentioned in the third section.' The model uses this information to generate a new, three-paragraph summary. In the context of the iterative improvement formula $y_{k+1} = \text{Refine}(x, y_k, \text{Feedback}(y_k))$ what does the term $y_k$ represent in this scenario?