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Create a WaitingFor context, and in the Context inspector, uncheck the box "Allows next actions". Any task in the WaitingFor context is now prevented from becoming an available or next action, but if it is at the top of a sequential project, it also prevents all subsequent tasks from being available. Awesome.

When you want to inspect WaitingFors, just focus on that context in the context view. You can also put due dates on waiting fors and sort them by due date in the context view.