datum on the in port is tested for. The former portion (mux, demux,
swap) determines the *action* to be taken if the condition tests true.
+\begin{verbatim}
action condition effect
------ --------- -------------------------------
swap false in1->out1 in2->out2
mux true in2->out1
demux false in1->out1
demux true in1->out2
+\end{verbatim}
In each case, the ship will wait for a datum to be available on all
input ports (and only those ports) mentioned in the appropriate row of