Stacked markup problem

[Jesse Engle]

I'm having an issue with stacking markup text in the correct order. The 
example input follows.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\version "2.12.3"

\new DrumStaff \new DrumVoice \drummode {
  \stemUp
  sn4:16_"L"_"R" ~ sn4->_"L"_"R"
  sn4:16_"R"_"L" ~ sn4->_"R"_"L"
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

What I wanted was for the first row of text to read "L L R R," and the 
second row to read "R R L L", but Lilypond seems to have a different 
means of choosing the display order. Is there any way to override this, 
or is it a bug?

Thanks in advance...Jesse