partcombine with 3-4 voices, flags not merged

[Eby Mani]

Hi All,

I'm typesetting some piano music, I find it easier to break the chords into 
different voices(e.g. rhA, rhB, rhC) and combine using \partcombine, instead of 
putting the chourds in a single voice (e.g. RH). 

However it doens't seem working fine with more than 2 voices on single staff. 
Following example does not combine "rhC" at all. 

The file i'm working on does combine noteheads and stems, except the flags. 
Instead of merging flags, it adds another set of flags in the same 
direction(sorry don't have access to the file i'm working on now, will provide 
screenshot once i get home).

The final result i'm looking for is that of "RH"'s with partcombine pairs 
without text.

Any snippets, commands to overcome this problem ?.

Thanks,
Eby

\version "2.18.2"
global = { \key c \major \time 2/4 }

rhA = \relative c' {
c8 e g c
}

rhB = \relative c' {
c8 c e e
}

rhC = \relative c' {
a8 a c c
}

RH = \relative c' {
<c a>8 <e c a> <g e c> <c e, c>
}

\score{
        <<
        \new Staff { \partcombine \sopranoA \sopranoB \sopranoC }
        \new Staff { \RH }
        >>
        \layout{}
        \midi{}
}