With the package, \_ in text mode (i.e., \textunderscore)
prints an underscore so that hyphenation of words either side
of it is not affected; a package option controls whether an
actual hyphenation point appears after the underscore, or
merely a break point. The package also arranges that, while in
text, '_' itself behaves as \textunderscore (the behaviour of _
in maths mode is not affected).
