(i) | \begin{equation} X \neq \emptyset \end{equation} |
(ii) | $0_S$ is an D39: Identity element in $S$ |
(1) | \begin{equation} y + x = 0_S \end{equation} | (D535: Left inverse element) |
(2) | \begin{equation} x + y = 0_S \end{equation} | (D536: Right inverse element) |