mat is a matrix of the evaluations on hyperprism D_n of a prismalinear extensions of the
columns. We “fold” it by evaluating the prismalinear polynomials at r in the univariate
variable Z.
For a sumcheck round, we want to compute the univariate polynomial s(X) = sum_{y \in H_{n-1}} \hat{f}(X, y). For this function, assume that \hat{f}(\vec x) = W(\hat{T}_0(\vec x), .., \hat{T}_{m-1}(\vec x)) for a sequence of \hat{T}_i where each \hat{T}_i consists of a
collection of MLE polynomials in n variables.
The univariate skip round 0: we want to compute the univariate polynomial s(Z) = sum_{x \in H_n} \hat{f}(Z, x). For this function, assume that \hat{f}(\vec z) = \hat\eps(\vec z) W(\hat{T}_0(\vec z), .., \hat{T}_{m-1}(\vec z)) for a sequence of \hat{T}_i where each
\hat{T}_i consists of a collection of prismalinear polynomials in n + 1 variables, with
degree < 2^{l_skip} in the first variable.