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\discretionary (generalized hyphenation point)

Synopsis:

\discretionary{pre-break}{post-break}{no-break}

Handle word changes around hyphens. This command is not often used in LaTeX documents.

If a line break occurs at the point where \discretionary appears then TeX puts pre-break at the end of the current line and puts post-break at the start of the next line. If there is no line break here then TeX puts no-break

In ‘difficult’ the three letters ffi form a ligature. But TeX can nonetheless break between the two f’s with this.

di\discretionary{f-}{fi}{ffi}cult

Note that users do not have to do this. It is typically handled automatically by TeX’s hyphenation algorithm.