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

file:./
               state = STATIC
          created by   root:
            supplies   file:data.txt
            supplies   file:optional.txt
            supplies   file:plan.py
            supplies   file:work.out
            supplies   file:work.py
            supplies   step:./plan.py
            supplies   step:./work.py
            supplies   step:cp -aT data.txt optional.txt

file:plan.py
               state = STATIC
          created by   root:
            consumes   file:./
            supplies   step:./plan.py

step:./plan.py
               state = SUCCEEDED
          created by   root:
            consumes   file:./
            consumes   file:plan.py
             creates   file:data.txt
             creates   file:work.py
             creates   step:./work.py
             creates   step:cp -aT data.txt optional.txt

file:data.txt
               state = STATIC
          created by   step:./plan.py
            consumes   file:./
            supplies   step:cp -aT data.txt optional.txt

file:work.py
               state = STATIC
          created by   step:./plan.py
            consumes   file:./
            supplies   step:./work.py

step:cp -aT data.txt optional.txt
               state = SUCCEEDED
           mandatory = REQUIRED
          created by   step:./plan.py
            consumes   file:./
            consumes   file:data.txt
             creates   file:optional.txt
            supplies   file:optional.txt

file:optional.txt
               state = BUILT
          created by   step:cp -aT data.txt optional.txt
            consumes   file:./
            consumes   step:cp -aT data.txt optional.txt
            supplies   step:./work.py

step:./work.py
               state = SUCCEEDED
          created by   step:./plan.py
            consumes   file:./
            consumes   file:optional.txt [amended]
            consumes   file:work.py
             creates   file:work.out
            supplies   file:work.out

file:work.out
               state = BUILT
          created by   step:./work.py
            consumes   file:./
            consumes   step:./work.py
