rewrite [Nat.sub_succ, Nat.pred_eq_sub_one]
case succ
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
⊢ trunc n (trunc n (b - Nat.succ w) - a) = trunc n (b - Nat.succ w) - a
case succ
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
⊢ trunc n (trunc n (b - w - 1) - a) = trunc n (b - w - 1) - a