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:cat test2.txt
             product   step:echo test1 > test1.txt
             product   step:echo test2 > test2.txt

step:echo test1 > test1.txt
               state = PENDING
                need = OPTIONAL
             creator   step:./plan.py
             product   file:test1.txt
                sink   file:test1.txt

file:test1.txt
               state = PLANNED
             creator   step:echo test1 > test1.txt
              source   step:echo test1 > test1.txt

step:echo test2 > test2.txt
               state = SUCCEEDED
                need = DEFAULT (implied by sinks > OPTIONAL)
             creator   step:./plan.py
             product   file:test2.txt
                sink   file:test2.txt

file:test2.txt
               state = BUILT
             creator   step:echo test2 > test2.txt
              source   step:echo test2 > test2.txt
                sink   step:cat test2.txt

step:cat test2.txt
               state = SUCCEEDED
                need = DEFAULT
             creator   step:./plan.py
              source   file:test2.txt
