\begin{circuslaw}{extend-sync-par}
\begin{circusaction*}
  A_1 \lpar ns_1 | cs | ns_2 \rpar A_2 ~ \equiv ~ A_1 \lpar ns_1 | cs \cup cs' | ns_2 \rpar A_2
  \also
  \provided \; cs' \cap (\usedC(A_1) \cup \usedC(A_2)) = \emptyset
\end{circusaction*}
\end{circuslaw}
