"Simon Marlow"
Hard to tell, but from the code above it looks like stage1 would be retained because it might need to be output after stage3. Could you perhaps move the output of stage1 up the pipeline?
Right. Stage1 is (indirectly) needed in the IO operation in the middle of the pipe, and at that time, the system hasn't evaluated the case at the end, and doesn't know whether it will be needed for the IO at the end of the pipeline. I've been massaging the code and profiling it, and it seems moving the output as far forward in the pipe as possible solved the problem. Thanks!
If you suspect there's a bug
I was a bug all right, but as usual, it was in my code, not yours :-) -kzm -- If I haven't seen further, it is by standing in the footprints of giants