hiding a class property behind a method

[Nynn007]

> The code uses the two objects through the A interface and x() 
> is a virtual function on that interface.
[...]
> Ali

The book "Programming in D" (r651) says in chapter "57.7 Using 
the subclass in place of the superclass", in the example about 
Clock and AlarmClock :

     void use(Clock clock) { ... }
(sic) "In other words, although use() uses the object as a Clock, 
the actual object may be an inherited type that behaves in its 
own special way".

Should'nt we understand that the first object is a B, the second 
object is a C and then should both behave like a B and a C, not 
like two A ?