[R] Question about S4

[Hun S. Tesatte]

Dear R-ers,

I don't understand the following, maybe someone will help me 
explain:

> setClasss('A')
[1] "A"
> new('a')
Error in new("a") : 
  trying to generate an object from a virtual class ("a")

> setClass('b', contains='a')
[1] "b"
> new('b')
An object of class “b”
<S4 Type Object>

In what way is B more concrete than A so that it's possible do 
instantiate B but not A? I don't quite get it. B adds nothing to 
nothing, and yet it's instantiable, while it's base is not. Makes 
no sense to me.

-- Hun