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:hop1.txt
             product   step:cp -p hop1.txt hop2.txt
             product   step:cp -p hop2.txt hop3.txt

file:hop1.txt
               state = CONFIRMED
             creator   step:./plan.py
                sink   step:cp -p hop1.txt hop2.txt

step:cp -p hop1.txt hop2.txt
               state = SUCCEEDED
                need = DEFAULT (implied by sinks > OPTIONAL)
             creator   step:./plan.py
              source   file:hop1.txt
             product   file:hop2.txt
                sink   file:hop2.txt

file:hop2.txt
               state = BUILT
             creator   step:cp -p hop1.txt hop2.txt
              source   step:cp -p hop1.txt hop2.txt
                sink   step:cp -p hop2.txt hop3.txt

step:cp -p hop2.txt hop3.txt
               state = SUCCEEDED
                need = DEFAULT
             creator   step:./plan.py
              source   file:hop2.txt
             product   file:hop3.txt
                sink   file:hop3.txt

file:hop3.txt
               state = BUILT
             creator   step:cp -p hop2.txt hop3.txt
              source   step:cp -p hop2.txt hop3.txt
