[tex-k] ligatures and explicit kerns

[Stephan Hennig]

Hi,

in the TeXbook, the answer to exercise 5.1 (the shelfful exercise)
contains a sentence

  Appendix H points out that ligatures are put into a hyphenated
  word that contains no “explicit kerns,” and an italic correction
  is an explicit kern.

I didn't find the place in appendix H the quote refers to, so pointers
would be appreciated.

Anyway, as I understand the quote, ligatures won't be inserted in words
that do contain explicit kerns.  That is, to suppress multiple ligatures
within a word, it should be enough to insert just /one/ italic
correction (between two of the ligature candidates).  Though, a test
turns out that this is not the case.

shelf\/fulfi
\bye

outputs the word with two separate letters f but the fi ligature
applied.  Could anybody please shed some light on the quote above?

Best regards,
Stephan Hennig