Linking Value Networks

In working on the manifesto and the architecture document it occurred to me
that we (or maybe it's just me) may be missing an essential feature in the
payment agent model.

If our payment agents are expected to talk to one another to negotiate the
terms of a payment, including the choice of payment scheme, then what do we
do when there is no common scheme between the participants?

Does the payment agent give up and say: "Sorry Alice, you can't pay Bob he
only accepts Visa, Bitcoin and ACH and you can only pay via MasterCard and
XRP, transaction aborted"?

If so then it seems we aren't solving anything. Our vision for
inter-connected value networks falls flat if our payment agents can only
facilitate a payment within existing closed networks.

Would I be correct in saying we need to consider that in many scenarios
there will be one or more intermediaries that "bridge" the two networks by
being plugged into both? How do we fit these brokers/intermediaries into
our architecture?

I think they are also payment agents of some sort but who do they interface
with? The sender, receiver, both? And, how does the payment flow between
Alice and Bob play out when this intermediary is required? At what point do
their agents say, "Oh dear, we don't have a common payment scheme we can
use, let's call Fred to act as a broker between your MasterCard and my Visa
accounts".

I'd like to discuss this on the call today as I think we need to figure it
out and put it in the document.

Adrian