chain\_sys\_ind($x$;${\it cmd}$.${\it input}$(${\it cmd}$);${\it from}$,${\it cmds}$.${\it update}$(${\it from}$;${\it cmds}$))
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$case $x$ of inl($x$) =$>$ ${\it input}$($x$) $\mid$ inr($x$) =$>$ ${\it update}$($x$.1;$x$.2)