rewrite [← tsub_le_iff_right]
case h
n x : ℕ
npos : 0 < n
xpos : 0 < trunc n x
⊢ trunc n x ≤ trunc n x - 1 + ulp n (trunc n x - 1)
case h
n x : ℕ
npos : 0 < n
xpos : 0 < trunc n x
⊢ trunc n x - ulp n (trunc n x - 1) ≤ trunc n x - 1