Fix temporary polyphony "for good" (was 'Re: Ties in complex polyphony')

[Kieren MacMillan]

Dear 'Pond (especially David K),

> I'd like to tie the common notes below:
> {
> <<{a1~}\\{f4 e}>>
> a1
> }
> 
> But I'm not sure I'm doing it correctly.

As this is a repeating, repetitive, periodic, cyclical, seasonal, perennial, 
regular, frequent, favo(u)rite, and recurrent trope on this list… Is it not 
possible — once and for all — to solve the "temporary polyphony" issue?

I believe the consensus, at one point, was that it would be best if Lilypond 
(by default) interpreted

    << a \\ b >>

as

  << { \voiceOne a } \\ \new Voice = "2" { \voiceTwo b } >> \oneVoice

The only points of debate were:
1. Should the new voice be explicitly named, and if so what name?; and
2. Is the parser/lexer/whatever in any shape to easily handle this "expansion" 
(if I recall correctly, Han-Wen suggested it might be difficult).

What say ye?
If I were a better programmer, with more time, I would try this myself — but 
I'm quite happy (as always) to pay for someone else to do it, if the bounty 
isn't too high.

Many thanks,
Kieren.