Specifies order without a hard more-recent-than dependency on everything after the |. So if the timestamp on "environ" is updated, that won't cause the "environ/lib" recipe to run, but if they both have to run then it ensures "environ" happens before "environ/lib".

It might not be necessary for this example, but I've found being more liberal with this feature and manually using "touch" has been more reliable in stopping unnecessary re-runs when the recipe target and dependency are directories instead of just files.