either

It becomes apparent I must next introduce the instruction "click the back link".

Together the functions "Forward" and "Back" define a phenomenon which occurs in other geometries, and in this one: the line.

"Forward" and "Back" are functions of another phenomenon in the domain of information: the page. The page is defined as a visible geometry which appears when a function of the class "get another page" is called, "Forward" and "Back" being example of functions of that class, and which, when it does appear, replaces, in the visual field the geometry of the page from which the function is called.

For the moment the reader is invited to "click either the forward or back link before clicking any other links on the screen (or off the screen)". This will be explained more fully at a later time.