Sorry about the newbie question but....

class A
  attr_accessor :a
end

class B < A
  attr_accessor :b
end

b = B.new
b.a = "some data"
b.b = "some more data"
a = b.?????

How do I set 'a' from 'b' so that a is of class A and not class B?  I 
don't want the extra info, I want just the subset of members found in 
the parent class.  Is there a way to cast?

TIA
 Michael Garriss