connection with p:choose.

Hi,
After having an error in some pipeline I was working on I manage to 
write the attached simple example.
Testing it with calabash or calumnet give an error.
Calabash: pipeline.xpl:31:43:No port named "result" on step named "choose"
Calumet: {http://www.w3.org/ns/xproc-error}XS0022: In all cases except 
the p:output of a compound step, it is a static error if the port 
identified by a p:pipe is not in the readable ports of the step that 
contains the p:pipe.
Pipe binding refers to a port that is not among the readable ports: 
Port{step=choose, port=result}

Not making the explicit connection work and solved my original problem 
but I was wandering if this error is intended or is a bug.