Instrument Transposition 'Leakage' To Other Instruments

[Will Oram]

Take the following snippet. There are two staves; the second one  
transposes in MIDI using \transposition a , whilst the first does  
not. Process it in 2.8.4 and listen to the generated MIDI. It clearly  
sounds like crap; this is because the first instrument is being  
transposed like the second.

\simultaneous {
                \new Staff <<
                        \new Voice { \InstrumentI }
                >>
                
                \new Staff <<
                        \partcombine
                                \relative c'' { \InstrumentIIA }
                                \relative g' { \InstrumentIIB }
                >>
}       

\layout {}

\midi { }

% end snippet

Now, I know why this osmosis happens. If both \InstrumentIIA and  
\InstrumentIIB include \transposition a , this problem occurs. Delete  
it from \InstrumentIIB, and \InstrumentI sounds at concert pitch.

My question is: is this a bug? This is not the first time this has  
happened to me when writing up a score, and it's taken many days to  
pinpoint the cause. If it's intentional, it's not logical.

I have to smile and remember
when you accuse me of that smell
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