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Let's disable page numbers:

\nopagenumbers
Hello world.
\bye

Page numbers appear if we redefine the nearest \par:

\begingroup\def\par{\endgraf\endgroup}
\nopagenumbers
Hello world.
\bye

If we redefine all \pars (\def\par{\endgraf}), everything is normal.

Why this happens?

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2 Answers 2

5

TeX is a macro expansion language; this means that it will use the current meaning of a macro and the current value of a variable when doing expansion and executing commands.

The meaning of \nopagenumbers is \footline{\hfil}; \footline is a token register, defined with \newtoks, so when \nopagenumbers is expanded, a local assignment to \footline is performed, that overrides (in the current group) the value it had before (which at start up is \hss\tenrm\folio\hss.

The value of \footline is used in the output routine, and TeX will use the value current when the output routine is called.

For instance, compare the two following pieces of code:

\begingroup\def\par{\endgraf\endgroup}
\nopagenumbers
Hello world.\vadjust{\vfill\penalty-10000 }\par

\begingroup\def\par{\endgraf\endgroup}
\nopagenumbers
Hello world.\par

You get two pages, the first with no page number, the second one having it. Why?

Because in the first case \endgraf causes the page builder to enter into action and the “recent contributions” are pushed into the main vertical list; the -10000 penalty causes the output routine to act when \endgroup has not yet been executed.

Also in the second case the recent contributions (here, the just completed paragraph) is pushed to the main vertical list as part of the action of \endgraf (which holds the primitive meaning of \par), but no page should be output, so \endgroup is executed and \footline gets its start up value because the group in which it was assigned a different value has ended.

Here's a picture (I used \vsize=2cm just to keep the picture small when showing the footline)

enter image description here

What happens with your code corresponds to the second example above, because the expansion of \bye starts with \par.

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9

The \nopagenumbers is only active for the current group, which is ended at the end of the current paragraph. But the page is shipped out afterwards, so page numbering is again active.

If you do the same thing in a paragraph which breaks over a page, you will get no page number on the page on which the paragraph begins:

As any dedicated reader can clearly see, the Ideal of practical reason is a
representation of, as far as I know, the things in themselves; as I have shown
elsewhere, the phenomena should only be used as a canon for our understanding.
The paralogisms of practical reason are what first give rise to the architectonic
of practical reason. As will easily be shown in the next section, reason would
thereby be made to contradict, in view of these considerations, the Ideal of prac-
tical reason, yet the manifold depends on the phenomena. Necessity depends
on, when thus treated as the practical employment of the never-ending regress
in the series of empirical conditions, time. Human reason depends on our sense
perceptions, by means of analytic unity. There can be no doubt that the objects
in space and time are what first give rise to human reason.

Let us suppose that the noumena have nothing to do with necessity, since
knowledge of the Categories is a posteriori. Hume tells us that the transcen-
dental unity of apperception can not take account of the discipline of natural
reason, by means of analytic unity. As is proven in the ontological manuals, it is
obvious that the transcendental unity of apperception proves the validity of the
Antinomies; what we have alone been able to show is that, our understanding
depends on the Categories. It remains a mystery why the Ideal stands in need
of reason. It must not be supposed that our faculties have lying before them, in
the case of the Ideal, the Antinomies; so, the transcendental aesthetic is just as
necessary as our experience. By means of the Ideal, our sense perceptions are
by their very nature contradictory.

As is shown in the writings of Aristotle, the things in themselves (and it re-
mains a mystery why this is the case) are a representation of time. Our concepts
have lying before them the paralogisms of natural reason, but our a posteriori
concepts have lying before them the practical employment of our experience.
Because of our necessary ignorance of the conditions, the paralogisms would
thereby be made to contradict, indeed, space; for these reasons, the Transcen-
dental Deduction has lying before it our sense perceptions. (Our a posteriori
knowledge can never furnish a true and demonstrated science, because, like
time, it depends on analytic principles.) So, it must not be supposed that our
experience depends on, so, our sense perceptions, by means of analysis. Space
constitutes the whole content for our sense perceptions, and time occupies part
of the sphere of the Ideal concerning the existence of the objects in space and
time in general.

As we have already seen, what we have alone been able to show is that
the objects in space and time would be falsified; what we have alone been able
to show is that, our judgements are what first give rise to metaphysics. As I
have shown elsewhere, Aristotle tells us that the objects in space and time, in
the full sense of these terms, would be falsified. Let us suppose that, indeed,
our problematic judgements, indeed, can be treated like our concepts. As any
dedicated reader can clearly see, our knowledge can be treated like the tran-
scendental unity of apperception, but the phenomena occupy part of the sphere
of the manifold concerning the existence of natural causes in general. Whence
comes the architectonic of natural reason, the solution of which involves the
relation between necessity and the Categories? Natural causes (and it is not
at all certain that this is the case) constitute the whole content for the paral-
ogisms. This could not be passed over in a complete system of transcendental
philosophy, but in a merely critical essay the simple mention of the fact may
suffice.

Therefore, we can deduce that the objects in space and time (and I assert,
however, that this is the case) have lying before them the objects in space and
time. Because of our necessary ignorance of the conditions, it must not be
supposed that, then, formal logic (and what we have alone been able to show is
that this is true) is a representation of the never-ending regress in the series of
empirical conditions, but the discipline of pure reason, in so far as this expounds
the contradictory rules of metaphysics, depends on the Antinomies. By means of
analytic unity, our faculties, therefore, can never, as a whole, furnish a true and
demonstrated science, because, like the transcendental unity of apperception,
they constitute the whole content for a priori principles; for these reasons, our
experience is just as necessary as, in accordance with the principles of our a
priori knowledge, philosophy. The objects in space and time abstract from all
content of knowledge. Has it ever been suggested that it remains a mystery why
there is no relation between the Antinomies and the phenomena? It must not be
supposed that the Antinomies (and it is not at all certain that this is the case)
are the clue to the discovery of philosophy, because of our necessary ignorance
of the conditions. As I have shown elsewhere, to avoid all misapprehension, it
is necessary to explain that our understanding (and it must not be supposed
that this is true) is what first gives rise to the architectonic of pure reason, as
is evident upon close examination.

\begingroup\def\par{\endgraf\endgroup}
\nopagenumbers
The things in themselves are what first give rise to reason, as is proven in
the ontological manuals. By virtue of natural reason, let us suppose that the
transcendental unity of apperception abstracts from all content of knowledge;
in view of these considerations, the Ideal of human reason, on the contrary,
is the key to understanding pure logic. Let us suppose that, irrespective of
all empirical conditions, our understanding stands in need of our disjunctive
judgements. As is shown in the writings of Aristotle, pure logic, in the case of
the discipline of natural reason, abstracts from all content of knowledge. Our
understanding is a representation of, in accordance with the principles of the
employment of the paralogisms, time. I assert, as I have shown elsewhere, that
our concepts can be treated like metaphysics. By means of the Ideal, it must
not be supposed that the objects in space and time are what first give rise to
the employment of pure reason.

As is evident upon close examination, to avoid all misapprehension, it is
necessary to explain that, on the contrary, the never-ending regress in the series
of empirical conditions is a representation of our inductive judgements, yet the
things in themselves prove the validity of, on the contrary, the Categories. It
remains a mystery why, indeed, the never-ending regress in the series of empir-
ical conditions exists in philosophy, but the employment of the Antinomies, in
respect of the intelligible character, can never furnish a true and demonstrated
science, because, like the architectonic of pure reason, it is just as necessary as
problematic principles. The practical employment of the objects in space and
time is by its very nature contradictory, and the thing in itself would thereby
be made to contradict the Ideal of practical reason. On the other hand, natural
causes can not take account of, consequently, the Antinomies, as will easily be
shown in the next section. Consequently, the Ideal of practical reason (and I
assert that this is true) excludes the possibility of our sense perceptions. Our
experience would thereby be made to contradict, for example, our ideas,
\bye

empty - numbered

As can be seen, page numbering resumes on the following page since, again, page numbering is re-activated at the end of the paragraph when the group ends.

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