have bpos : 0 < b - w :=
  Nat.zero_lt_of_lt $ Nat.lt_pred_iff.mp $ Nat.sub_succ _ _ ▸ h
case succ.inl
n a b : ℕ
npos : 0 < n
hfb : trunc n b = b
hf : trunc n (b - a) = b - a
w : ℕ
h : 2 * a < b - Nat.succ w
ih : trunc n (trunc n (b - w) - a) = trunc n (b - w) - a
hfb₁ : trunc n (b - w) = b - w
⊢ trunc n (trunc n (b - w - 1) - a) = trunc n (b - w - 1) - a
case succ.inl
n a b : ℕ
npos : 0 < n
hfb : trunc n b = b
hf : trunc n (b - a) = b - a
w : ℕ
h : 2 * a < b - Nat.succ w
ih : trunc n (trunc n (b - w) - a) = trunc n (b - w) - a
hfb₁ : trunc n (b - w) = b - w
bpos : 0 < b - w
⊢ trunc n (trunc n (b - w - 1) - a) = trunc n (b - w - 1) - a