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.
.vpt
[b] ¶\n[.vpt]
¶Enable or disable vertical position traps per Boolean expression
b. They are enabled by default, and if b is omitted.
Vertical position traps are those set by the wh
request or by
dt
within a diversion. Vertical position trap enablement is
global. Its status 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.