bs-define-sort-function is a compiled Lisp function in `bs.el'.

(bs-define-sort-function NAME FUN &optional REGEXP-FOR-SORTING FACE)

Define a new function for buffer sorting in Buffer Selection Menu.
NAME specifies the sort order defined by function FUN.
A value of nil for FUN means don't sort the buffer list. Otherwise the
functions must have two parameters - the buffers to compare.
REGEXP-FOR-SORTING is a regular expression which describes the
column title to highlight.
FACE is a face used to fontify the sorted column title. A value of nil means
don't highlight.
The new sort aspect will be inserted into list `bs-sort-functions'.