`@next_action` waterfalls into indented regions. If the top level task that is selected to receive the `@next_action` label has subtasks, the same algorithm is used. The `@next_action` label is only applied to one task.
Parallel list processing
------
If a list name ends with `=`, the top level of tasks will be treated as parallel `@next_action`s.
The waterfall processing will be applied the same way as sequential lists - every parent task will be treated as sequential. This can be overridden by appending `=` to the name of the parent task.