rewrite [hfb]
case zero
n a b : ℕ
npos : 0 < n
hfb : trunc n b = b
hf : trunc n (b - a) = b - a
h : 2 * a < b - Nat.zero
⊢ trunc n (trunc n b - a) = trunc n b - a
case zero
n a b : ℕ
npos : 0 < n
hfb : trunc n b = b
hf : trunc n (b - a) = b - a
h : 2 * a < b - Nat.zero
⊢ trunc n (b - a) = b - a