Accessing owner object within composed_of block

Hi All,

In my Account model I have a composed_of declaration like this:

class Account
  composed_of :balance, ... do |value|
    Money.new(value, 'USD')
  end
end

I would like to modify this so instead of always creating a USD, the
currency can be inferred from the 'currency' field in account. That
is,

class Account
  composed_of :balance, ... do |value|
    Money.new(value, currency)
  end
end

Of course, this wouldn't work since Account#currency is not within the
block's scope. Is there anyway for me to access Account#currency
within the composed_of block? Something akin to has_many's
proxy_owner object.

Thanks,
Eric