[reactive] "Time travel" with withNextE

[Robin Green]

withNextE seems to allow occurrences' values to depend on occurrences
which haven't yet occurred.

Of course, it doesn't actually allow time travel. If the occurrence is
not yet computed, presumably it will block until it does.

Still, this is problematic theoretically. It allows "simulations"
to be constructed which contain reverse causality (if the simulation
makes all the input occurrences available immediately). Is this a case
of making the framework a little too powerful?

-- 
Robin