With the present API, if the user sets the radius of an area monitor to be less than the implementation-defined minimum radius the minimum radius will be used in place of the user defined radius.
The API should have a read only property so that client code can discover what the implementation-defined minimum radius is.

At present this may be possible (indirectly) by reading the radius after setting it and comparing the values, but we in the cases where the implementation-defined minimum radius is used instead of the users value we currently don't guarantee that this change will be propogated back to the radius() method.