Add a Dependent to this cell.
The cell's value and most recent change time
Get a List of the Dependents
Get the cell's value
Create a new Cell by applying the function to this cell
Notify dependents of a state change.
If the predicate cell changes, the Dependent will be notified.
Remove a Dependent to this cell.
Remove from all dependencies
Get a list of all the cells this Dependency depends on
The Cell notifies the Dependent of the dependency
The Cell notifies the Dependent of the removed dependency