I would like associate a label to each "case" in the
environment case.
Something like :
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\begin{equation}
abs (x) = \begin{cases}
x& \text{if $x>0$}\label{pos},\\
-x& \text{otherwise}\label{neg}.
\end{cases}
\end{equation}
In the previous formulas, case~\ref{pos} corresponds to ...
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