Merging notes and shiftOn

[Nick Payne]

In the documentation on collision resolution 
(http://lilypond.org/doc/v2.15/Documentation/notation/multiple-voices#collision-resolution), 
it states: "The \shiftOn command allows (but does not force) the notes 
in a voice to be shifted. When \shiftOn is applied to a voice, a note or 
chord in that voice is shifted *only* if its stem would otherwise 
collide with a stem from another voice, and only if the colliding stems 
point in the same direction." (the emphasis on *only* is mine).

However, in the example below, where I have to use shiftOff in the 
middle voice (to get the two F notes to merge) and shiftOn in the treble 
voice (to prevent the merged half notes from having their heads filled 
in), the two A half notes in the treble voice *are* being shifted, 
though from my reading of the sentence quoted above, the shift is not 
needed.

\version "2.15.38"

treble = \relative c''' {
    \shiftOn
    a2 a
}

bass = \relative c' {
    f2 f
}

middle = \relative c' {
    \shiftOff f8[ c'] b c f,[ c'] b c
}

\score {
    \new Staff {
        \clef treble
        \mergeDifferentlyHeadedOn
<<
            \context Voice = "1" { \voiceOne \treble }
            \context Voice = "2" { \voiceTwo \bass }
            \context Voice = "3" { \voiceThree \middle }
>>
    }
    \layout { }
}

test.png
Description: PNG image