Creates a reactive page. A reactive page is defined by a function that is called once the participant reaches a given location in the timeline. This function must be idempotent, i.e. calling it multiple times should have the same effect as calling it once).
reactive_page(fun, next_elt = TRUE)
Function that returns a
(Logical scalar) Whether to go to the next element
in the timeline once this page is completed.
This will typically be