root:
             product   file:plan.py
             product   step:./plan.py

file:plan.py
               state = CONFIRMED
             creator   root:
                sink   step:./plan.py

step:./plan.py
               state = SUCCEEDED
                need = PLAN
           using_env = STEPUP_PATH_FILTER [dynamic]
             creator   root:
              source   file:plan.py
             product   file:data.txt
             product   file:work.py
             product   step:./work.py
             product   step:cp -p data.txt optional.txt

file:data.txt
               state = CONFIRMED
             creator   step:./plan.py
                sink   step:cp -p data.txt optional.txt

file:work.py
               state = CONFIRMED
             creator   step:./plan.py
                sink   step:./work.py

step:cp -p data.txt optional.txt
               state = SUCCEEDED
                need = DEFAULT (implied by sinks > OPTIONAL)
             creator   step:./plan.py
              source   file:data.txt
             product   file:optional.txt
                sink   file:optional.txt

file:optional.txt
               state = BUILT
             creator   step:cp -p data.txt optional.txt
              source   step:cp -p data.txt optional.txt
                sink   step:./work.py

step:./work.py
               state = SUCCEEDED
                need = DEFAULT
           using_env = STEPUP_PATH_FILTER [dynamic]
             creator   step:./plan.py
              source   file:optional.txt [dynamic]
              source   file:work.py
             product   file:work.out
                sink   file:work.out

file:work.out
               state = BUILT
             creator   step:./work.py
              source   step:./work.py
