Re: [paymentrequest] Should well-known identifiers be used for ubiquitous payment methods (#35)

> The API overall would be grounded ... to a latest version potentially if the application prefers that approach

This would be a disaster if a term ever changed it's range (allowable values) or semantic meaning (and we have examples of where that has happened in the past).

You can't have a message that is sent from a system and be consumed by two applications w/o having ONE clear mechanism for interpreting that message. If a v1 system sends the same message to a v2 system and a v3 system, the message could be interpreted by the v2 and v3 system differently (to catastrophic effect, since these are payment systems). Similarly, a v2 and v3 response without a known context being processed by a v1 system could result in something similarly terrible happening.

If you're not going to ground the message to a context, you have to ground the message to a version. 

A context grounds a message to a version explicitly and formally.

A version number grounds a message to a version explicitly, but informally (the message could still be interpreted by two systems differently, especially when the payload of the message can be extended by the sender).

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