apply trunc_eq_trunc_of_le_of_lt npos
n x : ℕ
npos : 0 < n
xpos : 0 < trunc n x
⊢ trunc n x = trunc n (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 ≤ trunc n x - 1 + ulp n (trunc n x - 1)

case k
n x : ℕ
npos : 0 < n
xpos : 0 < trunc n x
⊢ trunc n x - 1 + ulp n (trunc n x - 1) < next n (trunc n x)