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   step:cp -v first.txt second.txt

step:cp -v first.txt second.txt
               state = PENDING
                need = DEFAULT
             creator   step:./plan.py
              source   (file:first.txt)
             product   file:second.txt
             product   file:third.txt
                sink   file:second.txt
                sink   file:third.txt

(file:first.txt)
               state = UNDECLARED
                sink   step:cp -v first.txt second.txt

file:second.txt
               state = PLANNED
             creator   step:cp -v first.txt second.txt
              source   step:cp -v first.txt second.txt

file:third.txt
               state = VOLATILE
             creator   step:cp -v first.txt second.txt
              source   step:cp -v first.txt second.txt
