Corollary: No second-order repair of the fixed first response

A correction beginning at order $e^2$ cannot remove the nonzero order-$e$ defect of Proposition prop:ramp-defect while the ramp and first-order prescription remain fixed.

Proof. Equality of formal series modulo $e^3$ requires equality of their order-$e$ coefficients. Adding an order-$e^2$ term does not change that coefficient.