Path: utzoo!mnetor!uunet!munnari!otc!metro!basser!elecvax!agsm!bobm
From: bobm@agsm.unsw.oz (Robert Marks)
Newsgroups: comp.text
Subject: Re: troff query: tilde and ~ in eqn with .tr ~
Message-ID: <568@agsm.unsw.oz>
Date: 23 Mar 88 08:31:30 GMT
Organization: Australian Graduate School of Management
Lines: 31
Summary: answer: use a sed filter between eqn and ditroff

Thanks to Reinhard Foessmeier <foessmeier@infovax.informatik.tu-muenchen.dbp.de>
the problem is solved.
   The problem was how to print ~ as a diacritical in eqn (using the tilde
function) while also using ~ as an unpaddable space in both ordinary text
and in eqn (where it has a special status).
   It is now clear to me that with .tr ~ on, no ~s can be printed.
(I had wondered whether eqn mightn't turn off the .tr somehow.  But
examination of diacrit.c in the eqn sources showed that it doesn't.
BTW, the examination also showed me that utilde is a supported but
undocumented eqn function, which prints understrike tildes: try x utilde.)
   Reinhard suggested a sed filter between eqn and ditroff to take advantage
of the observation that eqn adds tilde diacriticals to equations in a manner
which enables isolation of these while all other remaining ~s in the text
are stream-edited to spaces.  (eqn substitutes spaces for ~s within
equations, so these have already disappeared downstream of eqn.)
   I used the filter
	eqn file | sed '/^\.ds/!s/~/ /g' | ditroff ...
which substitutes a space for all occurrences of ~ except those
in lines beginning .ds  Since I found that eqn was putting its diacritical
tildes in .ds lines:
	.ds 13 \v'-.1m'\s+1~\s0\v'.1m'
  So now the problem is solved.

Bob Marks
Australian Graduate School of Management, University of
New South Wales, PO Box 1, Kensington, NSW 2033, Australia.

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