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\begin{document}

\begin{subequations}
\label{eq:css:system}
\begin{align}
A &= a.b.c.A \label{eq:agent:a} \\
B &= \bar{a}.B + \bar{b}.B \label{eq:agent:b} \\
C &= (A\mid B) \backslash \{a,b\} \label{eq:agent:c}
\end{align}
\end{subequations}

\begin{align}
A &= a.b.c.A \tag*{powtórzenie \eqref{eq:agent:a}} \\
B &= \bar{a}.\bar{b}.B\label{eq:agent:b2}\tag{\ref{eq:agent:b}'} \\
C &= (A\mid B)\backslash\{a\}\label{eq:agent:c2}\tag{\ref{eq:agent:c}'}
\end{align}

\end{document}