I'm typesetting the attached music using four voices. However, a shifted quarter note refuses to merge with the half-note from other voice. I suspect it has to do with both having same stem direction, but how does one tell LP to merge anyway? Force-shifting notecolumn didn't seem to do the trick.
\version "2.18.2"
\relative c' {
\key g \minor
<<
{ \stemDown \hideNotes c8_~ \unHideNotes c2 }
\\
{ \stemUp c8[ d fis bes a] }
\\
{ \mergeDifferentlyHeadedOn \stemUp s8 d,2 }
\\
{ \mergeDifferentlyDottedOn s4 fis4. }
>>
}