how to add barre indications to automatic fret diagrams?

[pls]

Hi all,

LilyPond’s automatic fret diagrams really are a very helpful feature! More 
often than not it helps to not having to predefine and store a diagram in a 
lookup table.  But it has one little drawback: it doesn’t automatically include 
a barre indication when several strings are to be covered by one finger.  The 
default diagram actually looks a bit as if six fingers were needed to play this 
chord (see example).  

\version "2.19.15"
{
  \new FretBoards {
    <f,-1 c-3 f-4 a-2 c'-1 f'-1>
  }
}

I have had a look at the \override options of the FretBoard and many details 
are customizable but of course it’s not possible to override a barre indication 
that isn’t there.  Now, is there a known way to (manually) add barre 
indications to automatic fret diagrams or do I have to use predefined fret 
diagrams for barre chords in general?

tia
patrick