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5.28.1 Vertical Position Traps
------------------------------

A "vertical position trap" calls a macro when the formatter's vertical
drawing position reaches or passes, in the downward direction, a certain
location on the output page or in a diversion.  Its applications include
setting page headers and footers, body text in multiple columns, and
footnotes.

 -- Request: .vpt [flag]
 -- Register: \n[.vpt]
     Enable vertical position traps if FLAG is non-zero or absent;
     disable them otherwise.  Vertical position traps are those set by
     the 'wh' request or by 'dt' within a diversion.  The parameter that
     controls whether vertical position traps are enabled is global.
     Initially, vertical position traps are enabled.  The current value
     is stored in the '.vpt' read-only register.

     A page can't be ejected if 'vpt' is set to zero; see *note The
     Implicit Page Trap::.

* Menu:

* Page Location Traps::
* The Implicit Page Trap::
* Diversion Traps::