forest is an example that shows that pipeline construction is normal Go code -- the pipeline "forest" is created recursively and uses a global variable -- and that a pipeline may contain non-connected parts.
The pipeline generated has the shape of a forest where the output of each singleton leaf is flattened together over several rounds. This is most clearly seen via a visual representation of the pipeline, such as the one produced by the 'dot' runner.
Running the pipeline logs "1", "2", "3", etc for each leaf in the forest. Note that different runners may produces different or non-deterministic orders.