theorem discountGap_pos {D₀ δ_gap τ : ℝ} (hD : 0 < D₀)
(hδ : 0 ≤ δ_gap) (hτ : 0 ≤ τ) :
0 < discountGap D₀ δ_gap τ := by
simp only [discountGap]
have : 0 ≤ δ_gap * τ := mul_nonneg hδ hτ
linarithGrowth and Dynamic Tax Revenue (Layer 4-5 of Macro Extension)