[...]
a) replace the current mechanism by a command
(define-bar-line <shortcut>
(<bar-glyph> :prebreak <prebreak-glyph>
:postbreak <postbreak-glyph>
:span-bar <span-glyph>
:prebreak-span <pre-span>
:postbreak-span <post-span>))
where :prebreak, :postbreak, :spanbar, :prebreak-span and :postbreak-span
arguments are optional,and we define (in quasi-Lua syntax :-)
<prebreak-glyph> = <prebreak-glyph> or <bar-glyph>
<postbreak-glyph> = <postbreak-glyph> or #f
<span-glyph> = <span-glyph> or <bar-glyph>
<pre-span> = <pre-span> or <prebreak-glyph>
<post-span> = <post-span> or
<postbreak-glyph>:postbreak-:post::postbreak-span postbreak-span
break-span span