Can Span_bar_engraver only print at linebreaks?

[Graham Percival]

I'd like to connect staffs at the beginning (default) and end of each 
line, but
not in the middle.  Can any of the variables in Span_bar_engraver do 
this?
I'm not certain if print-fuction is designed to handle this kind of 
thing or not.


\score {
<<
  \new Staff { c'1 c' c' }
  \new Staff { c'1 c' c' }
>>
\layout{ \context { \Score
  \consists Span_bar_engraver
}}
}