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 [flag] ¶\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.