Function ring_target 

pub fn ring_target<F>(
    plan: &RingSwitchPlan,
    claims: &[Flat<F>],
    eta_tower: F,
    r_mix: &[Block128],
    shifted_claims: bool,
) -> Block128
where
    F: HardwareField + Into<Block128>,

Reconstructs the sumcheck’s initial claim from the claimed virtual evals, in the tower basis. Ring units contribute eta·Σ_u eq(r'',u) ŝ_u; whole units contribute eta·c'.