\begin{circuslaw}{extract-var-prefix}
\begin{circusaction*}
  c~?~x \then (\circvar v : T \circspot A) ~ \equiv ~ \circvar v : T \circspot c~?~x \then A
  \also
  \provided \; \mbox{$x$ and $v$ are distinct variables}
\end{circusaction*}
\end{circuslaw}
