[Haskell-cafe] ArrowChoice for simulation arrow (Sim)

Hi, In John Hughes\' paper \"Programming with arrows\" [1] Section 5, the simulation arrow is defined (Sim). All the examples went well. However, when I want to use it beyond circuit simulation and I added a \"if-then-else\" line, GHC is asking to define ArrowChoice for Sim. Does anybody have the ArrowChoice definition for Sim ? Thanks, Steve [1] http://www.cs.chalmers.se/~rjmh/afp-arrows.pdf