Sorting on compound keys?

[Tim Landscheidt <tim@tim-landscheidt.de>]

Hi,

sometimes I want to sort unified diffs of CSV files (sepa-
rated by tabs (here: \t)):

| +A 1\t1\tx
| +A 1\t2\ty
| +B 2\t3\tz
| -A 1\t1\tx
| -B 2\t2\ty
| -B 2\t3\tz

by the second column, then the first column, then "+" vs.
"-". Unfortunately, it seems that sort-regexp-fields doesn't
allow more than one match field as a key. sort-fields
doesn't work either as it requires the fields to be sur-
rounded by white space (no "+" vs. "-") and doesn't allow
white space inside the fields.

  Is there any function in vanilla Emacs (23.1.1) that I
missed? I looked at pimping sort-regexp-fields, but it seems
to me that sort-subr would have to be rewritten from scratch
to achieve sorting on compound keys.

Tim