apply tsub_le_tsub_right
n a b : ℕ
npos : 0 < n
hfa : trunc n a = a
le_ulp : b ≤ ulp n a
size_lt : 2 ^ (Nat.size a - 1) < a
⊢ 2 ^ (Nat.size a - 1) + ulp n a - b ≤ a - b
case h
n a b : ℕ
npos : 0 < n
hfa : trunc n a = a
le_ulp : b ≤ ulp n a
size_lt : 2 ^ (Nat.size a - 1) < a
⊢ 2 ^ (Nat.size a - 1) + ulp n a ≤ a