\shiftOn and friends

[Richard Shann]

In the following I have inserted \shiftOn and \shiftOff commands in a
vain attempt to get the <e g> chord in the second voice to be displaced
to the left of the a8 in voice one. I have even repeated the command
immediately before these notes, but to no avail. (I am hoping it will
fix the collision of the tie with the note head).
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\version "2.18.0"
MI = {\voiceOne \shiftOn a4.~  \shiftOn a8 b c'}
MII = { \voiceTwo \shiftOff a8 g^~  <e_~ g^~ >8 \shiftOff <e g> \skip 4 }

\new Staff   << 
 \new Voice \clef bass \time 6/8 \MI       
 \new Voice   \MII                    
>>

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Am I doing something wrong?

Richard Shann