New auxiliary macros: \bk@singleline, \bk@firstline, \bk@middleline and \bk@lastline
Revert "Revert "New macros: \bk@singleline, \bk@firstline, \bk@middleline and \bk@lastline""

This reverts commit eb8b3dacd9d169883000267f35bc58e7eadb6810.