pub fn compute_initial_sum(&self) -> Result<FieldElement<F>, ProverError> {
        sum_product_over_suffix(&self.factors, &[])
            .map_err(|e| ProverError::SummationError(format!("Error computing initial sum: {}", e)))
    }
Computes the initial claimed sum \( C = \\sum_{x \\in \\{0,1\\}^n} \\prod_i P_i(x) \).
crates/provers/sumcheck/src/prover.rs