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:a_input.txt
             product   file:b_input.txt
             product   step:cp -p a_input.txt a.txt
             product   step:cp -p b_input.txt b.txt

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

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

step:cp -p a_input.txt a.txt
               state = PENDING
                need = DEFAULT
             creator   step:./plan.py
              source   file:a_input.txt
             product   file:a.txt
                sink   file:a.txt

file:a.txt
               state = OUTDATED
             creator   step:cp -p a_input.txt a.txt
              source   step:cp -p a_input.txt a.txt

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

file:b.txt
               state = BUILT
             creator   step:cp -p b_input.txt b.txt
              source   step:cp -p b_input.txt b.txt
