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It might also be helpful to think about how you would write the two-line notation for \(h\text{.}\) We would have something like:

\begin{equation*} h=\twoline{1 \amp 2 \amp 3 \amp 4}{\amp a,c? \amp d \amp b}\text{.} \end{equation*}

There is nothing under 1 (bad) and we needed to put more than one thing under 2 (very bad). With a rule that is actually a function, the two-line notation will always “work”.

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