[Newbies] Tweak question

[itsme213]

I don't understand (what appears to me as) some duplication in Tweak's event
propagation mechanism in the bank-account tutorial 
http://tweak.impara.de/TECHNOLOGY/Tutorials/BankAccountTutorial/ : an ATM
hooked to a bank account and displaying its balance on its #balanceReadout.

It keeps the #balanceReadout widget current by hooking into 2 separate 
events
with 2 pieces of code:

onAccountChanged
 "Update the balance readout when the account changes"
 <on: accountChanged>
 self updateBalanceReadout

onAccountValueChanged
 "Update the balance readout when the account value changes"
 <on: valueChanged in: account>
 self updateBalanceReadout.

This seems unnecessary. If I simply declared the path to the target object
being observed:
   { #account #value }
It seems I have said enough to respond correctly to both #accountChanged and
#valueChanged. The rest could be done by Tweak behind the scenes.

Such a role-chain is a common scenario:
    A is watching the #x of the #y of the #z.
Hence A has to react to xChanged, yChanged, and zChanged (in the appropriate
corresponding target objects).

Similarly, given Tweaks real + virtual fields, I'm not sure why all the
on<X>Changed stuff needs to be explicit. Why not directly reference the
fields e.g. Isn't the underlying event behavior adequately determined
by something declarative along the lines of:

  { #balanceReadout #value } derivedFrom: { #account #value }

Is there already some better way to do this in Tweak, just not used in the
tutorial?

Thanks.