Reference to a fallback of the same service or null if no fallback.
If the user needs to abandon this instance, they should use this to clean up active resources (like timers) and to release memory that would stall the garbage collector from disposing this object. It should assign any object reference to undefined as a strong indicator that the object has been disposed.
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Assign to services that support a fallback version of itself, so the user can override the service with a special case, and the fallback handles the rest.