Vertical position traps perform an action when GNU troff
reaches or passes a certain vertical location on the output page or in a
diversion. Their 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.