can't make merge-differently-headed work

[Atte André Jensen]

The following .ly (hope my mailer doesn't screw up the line breaks...)
doesn't merge the note heads. Even at spots where it seems to work,
you'll notice on closer inspection that there are actually printed two
note heads slightly unaligned. What do I do wrong?

This is all with 1.6.4, BTW...

-- begin .ly --
RHa = \notes \relative c' \context Voice = RHa {
    \property Staff.NoteCollision \override #'merge-differently-headed =
##t
    \voiceOne
    | <c8. f c'> f''16 () f,8 <b,,8. e b'> g'''16 () g,8
    | <d,8. e a> a''16 ( <a,8 f'> <a8. ) e'> a''16 (<a,8 ) e'>
}

RHb = \notes  \relative c' \context Voice = RHb {
    \property Staff.NoteCollision \override #'merge-differently-headed =
##t
    \voiceTwo
    | <c4. f c'><b e b'>
    | <d e a> s
}

LHa = \notes  \relative c, \context Voice = LHa {
    \voiceThree
    \property Staff.NoteCollision \override #'merge-differently-headed =
##t
    | <d4. \arpeggio a' f'><e \arpeggio b' g>
    | <a \arpeggio e' a> \clef treble <a' \arpeggio e' a>
}


LHb = \notes  \relative c, \context Voice = LHb {
    \voiceFour
    \property Staff.NoteCollision \override #'merge-differently-headed =
##t
    | <d8 a' f'> d, () d' <e b' g> e, () e'
    | <a e' a> a, () a' <a' e' a> a'( <e') a>
}

\score {
    \context PianoStaff
    \notes <
        \context Staff = up <
            \time 6/8
            \key a \minor
            \clef treble
            \RHa
            \RHb
        >
        \context Staff = down <
            \time 6/8
            \key a \minor
            \clef bass
            \LHa
            \LHb
        >

    >
    \paper {
    }
}
-- end .ly --

Thanks in advance :-)

-- 
peace, love & harmony
Atte