[reactive] "Time travel" with withNextE

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