The problem of dynamic inheritance

[cyberbaixing]

Dear All,

I am a newer of SELF. I try to run the a program of dynamic 
inheritance. 
which is defined as follows:

globals _AddSlotsIfAbsent: (|
        a =().
        b =().
        c =().
|)

a _Define: (|
        f = ('a' printLine. p:b. f).
|)
 
b _Define: (|
        f = ( 'b' printLine ).
|)

c _Define: (|
        p*<- a.
        b = b.
        f = ('c' printLine. resend.f).
|)


But if I send the message f to slot a, the result is always:
"Self 14" a f.
a
No 'b' slot found in a slots object.

If I rewrite the slot a without the dynamic inheritance but replace 
with directed resends or undirected resends like this:

a _Define: (|
        p*<- b.
        f = ('a' printLine. b. f).
|)

The result is:
"Self 14" a f.
a
b
'b' 
Why the resends works but the dynamic inheritance not? Did I make any 
semantic mistakes in slot a's definition?

Furthermore, how to understand the meaning of "activation record"?  
In the above example, which one can be looked as the current 
activation record? I  know the slot a is the receiver of the message, 
the method f has a clone. The code of f is running in the context of 
clone. 'a'printLine prints out the string a, the code p:b.f is to 
change the parent of slot a to slot b. But where is the activation 
record here? Is it same as the activation object?

Thank you in advance.

Regards,
Xing