Expression for: foo is derived from Clock

[Brother Bill via Digitalmars-d-learn]

```Object foo``` is a class instance, perhaps null, perhaps a 
Clock or AlarmClock or Vegetable.

What is the expression to ask the boolean question:
  Is foo of type Clock or a type derived from Clock, such as 
AlarmClock?


I tried the obvious ```foo is Clock```, but this did not compile.
```
Error: type `Object` is not an expression
       bool isClock = (o is Clock)
                            ^
```