The definition of the active " character needs a couple of support
macros. The macro \allowhyphens is used make hyphenation of word
possible where it otherwise would be inhibited by TEX. Basically its
definition is nothing more than \nobreak \hskip 0pt plus 0pt.
\gdef\allowhyphens{\penalty\@M \hskip\z@skip}
Then a macro is defined to lower the Dutch left double quote to the same level as the comma. It prepares a low double opening quote in box register 0. This macro was copied form german.tex.
\gdef\set@low@box#1{%
\setbox\tw@\hbox{,} \setbox\z@\hbox{#1}
\dimen\z@\ht\z@ \advance\dimen\z@ -\ht\tw@
\setbox\z@\hbox{\lower\dimen\z@ \box\z@}
\ht\z@\ht\tw@ \dp\z@\dp\tw@}
The macro \set@low@box is used to define low opening quotes.
Since it may be used in arguments to other macros it needs to be protected.
\gdef\dlqq{\protect\@dlqq}
\gdef\@dlqq{{%
\ifhmode
\edef\@SF{\spacefactor\the\spacefactor}
\else
\let\@SF\empty
\fi
\leavevmode\set@low@box{''}
\box\z@\kern-.04em\allowhyphens\@SF\relax}}
For reasons of symmetry we also define "'. This command is defined
similar to \dlqq, except that the quotes aren't lowered to the baseline.
\gdef\@drqq{{%
\ifhmode
\edef\@SF{\spacefactor\the\spacefactor}
\else
\let\@SF\empty
\fi
''\@SF\relax}}
The original double quote character is saved in the macro \dq to keep
it available.
\begingroup \catcode`\"12
\gdef\dq{"}
\endgroup
The original definition of \" is stored as \dieresis. The resason
for this is that if a font with a different encoding scheme is used the
definition of \" might not be the plain TEX one.
\global\let\dieresis\"
In the Dutch language vowels with a dieresis or umlaut accent are treated
specially. If a hyphenation occurs before a vowel-plus-umlaut, the umlaut
should disappear. To be able to do this, the hyphenation break behaviour for
the five vowels, both lowercase and uppercase, could be defined first in
terms of \discretionary. But this results in a large
\if-construct in the definition of the active ".
As both Knuth and Lamport have pointed out, a user should not use " when
he really means something like ''. For this reason no distinction is
made between vowels and consonants. Therefore one macro, \@umlaut,
specifies the hyphenation break behaviour for all letters.
\def\@umlaut#1{%
\allowhyphens%
\discretionary{-}{#1}{\dieresis #1}%
\allowhyphens}
The last support macro to be defined is \dutch@active@dq.
\gdef\dutch@active@dq#1{%
\if\string#1`\dlqq{}%
\else\if\string#1'\drqq{}%
\else\if\string#1-\allowhyphens-\allowhyphens%
\else\if\string#1|\discretionary{-}{}{\kern.03em}%
\else\if\string#1i\allowhyphens\discretionary{-}{i}{\dieresis\i}%
\allowhyphens%
\else\if\string#1j\allowhyphens\discretionary{-}{j}{\dieresis\j}%
\allowhyphens%
\else \@umlaut{#1}\fi\fi\fi\fi\fi\fi}
The macro reads the next token and performs some appropriate action. If no
special action is defined, it will produce an umlaut accent on top of
argument 1.
The last definition needed is a replacement for \-. The new version of
\- should indicate an extra hyphenation position, while allowing other
hyphenation positions to be generated automatically. The standard behaviour of
TEX in this respect is very unfortunate for languages such as Dutch and
German, where long compound words are quite normal and all one needs is a means
to indicate an extra hyphenation position on top of the ones that TEX can
generate from the hyphenation patterns.
\def\-{\allowhyphens\discretionary{-}{}{}\allowhyphens}