promises mutex

[Kevin Ryde]

Is there a theory on having a mutex for each promise?

I see it has the effect of preventing two threads simultaneously
forcing, but I wonder if it'd be easier to allow that, just a single
mutex to protect the final store of the result.

This would be smaller and faster, and might even be beneficial for
some crazy cross-thread recursive forcing.  It'd be in keeping with
leaving most syncing up to the application (ie. allowing most anything
not actively harmful).