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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 The Implicit Page Trap.