apply ih
n a b : ℕ
npos : 0 < n
hfb : trunc n b = b
hf : trunc n (b - a) = b - a
w : ℕ
ih : 2 * a < b - w → trunc n (trunc n (b - w) - a) = trunc n (b - w) - a
h : 2 * a < b - Nat.succ w
⊢ trunc n (trunc n (b - w) - a) = trunc n (b - w) - a
n a b : ℕ
npos : 0 < n
hfb : trunc n b = b
hf : trunc n (b - a) = b - a
w : ℕ
ih : 2 * a < b - w → trunc n (trunc n (b - w) - a) = trunc n (b - w) - a
h : 2 * a < b - Nat.succ w
⊢ 2 * a < b - w