Question about exercise 5.1 in TeXbook

Question

The answer to the exercise states:

TEX will reinsert the ff ligature by itself after hyphenating shelf{}ful.
(Appendix H points out that ligatures are put into a hyphenated word that contains no
“explicit kerns”

But the following example shows that ligature is reinserted even if the word is not hyphenated:
hsize=70pt a shelf{}ful firefly
end

Which rule in appendix H explains that?

But if the first word is not used, the ligature is not reinserted:
hsize=70pt shelf{}ful firefly
end

egreg · Accepted Answer

Consider the example file
hsize=70pt a shelf{}ful firefly

hsize=70pt shelf{}ful firefly
end

Upon running TeX on it, we get on the terminal
Overfull hbox (7.58342pt too wide) in paragraph at lines 1--2
[]tenrm a shelf-ful fire-|

Overfull hbox (0.63898pt too wide) in paragraph at lines 3--4
[]tenrm shelfful fire-|

You can notice that in the second case the symbolic representation is different and the reason is that TeX never tries hyphenation on a word fragment that's not preceded by glue. This restriction is not used in LuaTeX and, indeed, if we run LuaTeX on the same file we get
Overfull hbox (7.58342pt too wide) in paragraph at lines 1--2
[]tenrm a shelf-ful fire-|

Overfull hbox (0.36119pt too wide) in paragraph at lines 3--4
[]tenrm shelf-ful fire-|

Once the word fragment has been subjected to trial hyphenation, ligatures are reconstituted. The TeXbook indeed recommends to use either the italic correction / or another explicit kern, in order to break ligatures. In the case of ff, kern0pt seems best with Computer Modern, but it may not be with another font.

Question about exercise 5.1 in TeXbook

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