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Polynomial Breakdown in proof of lower bounds on Discrete Log in the Generic Group

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In Shoup's proof of the hardness of discrete log in the generic group in this paper, he mentions that:

At any step in the game, the algorithm has computed a list $F_1,\dots,F_k$ of linear polynomials in $Z/p^t[X]$ along with a list of values $z_1,\dots,z_k$ in $Z/s$, and a list $\sigma_1,\dots,\sigma_k$ of distinct values in $S$.

The algorithm is initially given the encodings of $1,x$ and access to the group operation + inverses so it is clear that anything the algorithm computes can be expressed as a linear polynomial in $Z/n[X]$, where $n=p^t s$. However, I don't see how this breaks down into a linear polynomial in $Z/p^t[X]$ and a constant in $Z/s$.

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