rewrite [tsub_tsub_cancel_of_le hcb]
case inr
n a b c : ℕ
npos : 0 < n
hac : a ≤ c
hcb : c ≤ b
hfa : trunc n a = a
hfb : trunc n b = b
hfc : trunc n c = c
hf : trunc n (b - a) = b - a
hgt : c > 2 * a
⊢ 2 * a < b - (b - c)
case inr
n a b c : ℕ
npos : 0 < n
hac : a ≤ c
hcb : c ≤ b
hfa : trunc n a = a
hfb : trunc n b = b
hfc : trunc n c = c
hf : trunc n (b - a) = b - a
hgt : c > 2 * a
⊢ 2 * a < c