BEGIN token marks the entrypoint of graphs, and all extractors will be connected to this node.
Without this, it would be impossible for an execution to actually start anything, as it’s the marker that tells Bonobo which node to actually call when the execution starts.
NOT_MODIFIED is a special value you can return or yield from a transformation to tell bonobo to reuse the input data as output.
As a convention, all loaders should return this, so loaders can be chained.
Shortcut for “empty tuple”. It’s often much more clear to write (especially in a test) write(EMPTY) than write(()), although strictly equivalent.