What is the effect of calling then() on several shared_future instances?

[coryan]

Sorry if this is the wrong forum, but the specification seems to be silent on the topic and this is there the specification lives...

For a regular future one cannot really call then() multiple times because the first call invalidates the future (and effectively only one future can point to the shared state).
For shared_future multiple instances can point to the same shared state, and the application may call then() on any of them. There is no description on what the effect should be. Should all continuations be called? Only one be set?