Prove: Given an $m \times n$ matrix $A$ and a vector $b \in \mathbb{R}^m$,
EITHER
(i) there exists a vector $x \in \mathbb{R}^n$ with $x \geq 0$ and $Ax = b$,
OR
(ii) there exists a vector $y \in \mathbb{R}^m$ with $y^\top A \geq \mathbf{0}$ and $y^\top b < 0$,
NOT BOTH.