Compute sort.key$ for entries

[digitalheir]

Also see #3

When sorting, BibTEX computes a string, named
sort.key$, for each entry. The sort.key$ string is an (often long) string defining the order
in which entries will be sorted. To avoid any ambiguity, sort.key$ should only contain alphanumeric
characters. Classical non-alphanumeric characters23, except special characters, will
be removed by a BibTEX function named purify$. For special characters, purify$ removes
spaces and LATEX commands (strings beginning with a backslash), even those placed between
brace pairs. Everything else is left unmodified. For instance, t^ete, t{^e}te and t{^{e}}te
are transformed into tete, while tête gives tête; Bib{\TeX} gives Bib and Bib\TeX becomes
BibTeX. There are thirteen LATEX commands that won’t follow the above rules: \OE, \ae, \AE,
\aa, \AA, \o, \O, \l, \L, \ss. Those commands correspond to ı, , œ, Œ, æ, Æ, å, Å, ø, Ø, ł, Ł,
ß, and purify$ transforms them (if they are in a special character, in i, j, oe, OE, ae, AE, aa,
AA, o, O, l, L, ss, respectively.

http://tug.ctan.org/info/bibtex/tamethebeast/ttb_en.pdf