  We introduce bisimulations for the logic $ITL^e$ with `next', `until' and
`release', an intuitionistic temporal logic based on structures equipped with a
partial order used to interpret intuitionistic implication and a monotone
function used to interpret the temporal modalities. Our main results are that
`eventually', which is definable in terms of `until', cannot be defined in
terms of `next' and `henceforth', and similarly that `henceforth', definable in
terms of `release', cannot be defined in terms of `next' and `until', even over
the smaller class of here-and-there models.
