- and + have special treatment in numbers or
terms like C++, else they are processed as span glue.
. and ' are always processed as span
glue.
# is processed as a letter at the beginning (hashtag) or end (c#)
of a word, else as white space.
The underscore (_) is processed as span glue except if the
underscoreasletter index configuration parameter is set, in which case
it is treated as a regular letter.
