Two notes with the same notehead but different accidentals should not be merged

[Yuan Ye]

> I'm not top posting.

In the following minimal example, none of the four solutions is acceptable.
The last one may be the least confusing, but still, in choral music, I
definitely want to show clearly that the upper voice is g and the lower
voice is gis, thus I have to use multiple voices.
 
\version "2.16.2"

\new Voice \relative g' {
        <<
        { \voiceOne g g! }
        \new Voice { \voiceTwo gis gis! }
        >>
        \oneVoice <g gis> <g! gis!>
}