cases le_or_gt c (2 * a) with
| inl hle => exact sterbenz hfc hfa hac hle
| inr hgt =>
  rewrite [← hfc]
  rewrite [← tsub_tsub_cancel_of_le hcb]
  apply s₁' npos hfb hf
  rewrite [tsub_tsub_cancel_of_le hcb]
  exact hgt
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
⊢ trunc n (c - a) = c - a
no goals