Re: [Groff] Explanation for .BP ?

[Werner LEMBERG]

> To generate a page break within a table, one is supposed to use
> 
> .de BP
> .  ie ’\\n(.z’’ .bp \\$1
> .  el \!.BP \\$1
> ..
> 
> Could some kind soul explain the working of this macro?  The else
> branch looks like a recursive call.  And why does plain .bp not
> work?

A table is normally built-up within a diversion, and page breaks don't
work within a diversion.  This diversion gets emitted right before the
table definition ends.

The macro BP first tests whether we are within a diversion.  The
number register \n[.z] contains the name of the current diversion
(yes, a number register which contains a string); if it is empty, we
are not within a diversion.  While defining a table, \n[.z] is *not*
empty, so the macro embeds a call to `.BP' uninterpreted into the
diversion (due to the leading `\!' escape).  As soon as the diversion
is emitted, it now interprets the `.BP' macro.  Since \n[.z] is empty
this time, the call to `.bp' gets executed.

Note that `.BP' even works with GNU tbl's option `nokeep'.


     Werner