# Question about exercise 12.17 in TeXbook

This is exercise 12.17

(This is a sequel to exercise 12.2, but it's trickier.) Describe the result of \line{\hfil A puzzle.\hfilneg}

This is exercise 12.2

Describe the result of \line{\hfil\hfil What happens now?\hfil} \line{\hfill\hfil and now?\hfil}

... (In this case, TeX's rule about infinite glue differs from what you would get in the limit if the value of 1 fil were finite but getting larger and larger. The true limiting behavior would stretch the text A puzzle.' in the same way, but it would also move that text infinitely far away beyond the right edge of the page.)

Example from the question is equivalent to this:

\hbox to\hsize {\hskip 0pt plus 1fil A puzzle.\hskip 0pt plus -1fil}


How in terms of this does one interpret the words "if the value of 1 fil were finite" and "true limiting behavior"?

\hfilneg isn't a minus value but rather a negative plus value:

\hbox to\hsize {\hskip 0pt plus 0pt A puzzle.\hskip 0pt plus -0pt}

\hbox to\hsize {\hskip 0pt plus 10pt B puzzle.\hskip 0pt plus -10pt}

\hbox to\hsize {\hskip 0pt plus 100pt C puzzle.\hskip 0pt plus -100pt}

\hbox to\hsize {\hskip 0pt plus 1000pt D puzzle.\hskip 0pt plus -1000pt}

\hbox to\hsize {\hskip 0pt plus 1fil E puzzle.\hskip 0pt plus -1fil}

\hbox to\hsize {\hfil F puzzle.\hfilneg}

\bye
`

Note the last two examples E and F are the same, but the point is they do not correspond to the limiting case of A-D as the finite skip gets larger.

• Fixed the question. BTW, should "was" be used instead of "were" in the answer? (because the concept of 1fil is regarded here as a singular thing) – Igor Liferenko Aug 26 '15 at 23:09
• @IgorLiferenko were is OK here it's not singular/plural but rather the conditional would .. if ... were (but don't ask an Englishman about English grammar:-) – David Carlisle Aug 26 '15 at 23:48