This class represents a hyphen. A 'full' hyphen is made of 3 parts:
the pre-break text, post-break text and no-break. If no line-break
is generated at this position, the no-break text is used, otherwise,
pre-break and post-break are used. Typically, pre-break is equal to
the hyphen character and the others are empty. However, this general
scheme allows support for cases in some languages where words change
spelling if they're split across lines, like german's 'backen' which
hyphenates 'bak-ken'. BTW, this comes from TeX.