proposition: Theorem P_eq_Delta_Q : c.CT c.
proof: Proof. move=> n k; rewrite /c.CT /c.not_D => notD. rewrite /c.P_horner /c.P_seq /punk.horner_seqop /= /c.Q_flat. do 2! (rewrite c_Sn ; last by rewrite /c.precond.Sn; lia). rewrite c_Sk; last by rewrite /c.precond.Sk; lia. rewrite /c.P_cf0 /c.P_cf1 /c.P_cf2 /c.Q_cf0_0 /c.Sn_cf0_0 /c.Sk_cf0_0. rewrite !int.shift2R. by field; ring_lia. Qed.