feature req: slur versions of \repeatTie and \laissezVibrer

[Mark Polesky]

feature req: slur versions of \repeatTie and \laissezVibrer
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When a (non-initial) volta alternative starts with a note 
tied from the end of the volta, we can use \repeatTie to 
correctly display the half-tie, but it seems there's no way
of displaying a half-slur when the alternative contains a 
note slurred from the volta (ie. there's no \repeatSlur).

One issue would be that the half-slur would need to look 
backwards for the beginning of the alternative because the
slurred-to note wouldn't necessarily be the first note of 
the alternative.
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Similarly, when a (non-final) volta alternative ends with a
note tied back to the beginning of the volta, we can use 
\laissezVibrer to fake a half-tie. There seems to be no 
trick for faking a half-slur when the alternative contains 
a note slurred back to the beginning of the volta.

In this instance the issue would be that the half-slur 
would need to look forward to find the end of the 
alternative since the repeat sign doesn't necessarily 
follow the slurred note immediately -- the slur would need 
to go past other notes until the repeat sign is reached.

However, if an implementation were even plausible (I have 
no idea), I wouldn't propose \laissezVibrerSlur since 
that's nonsensical. Perhaps \repeatSlurRight?
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WARNING: the following may be preposterous.

Since the \laissezVibrer half-tie trick is semantically 
wrong, we could introduce \repeatTieRight. While we're in
that ballpark, we could introduce \laissezVibrerLeft -- I 
have seen French editions (Messiaen?) where chords held
across measures have left- and right-sided l.v. ties. And
then there are the phrasing slurs...

Assuming these additions, we're left with:

   \repeatTie
   \repeatTieRight
   \repeatSlur
   \repeatSlurRight
   \repeatPhrasingSlur
   \repeatPhrasingSlurRight
   \laissezVibrer
   \laissezVibrerLeft

Amid this new constellation of commands, perhaps \repeatTie,
\repeatSlur, and \repeatPhrasingSlur would be less vague if
they were called \repeatTieLeft, \repeatSlurLeft, and 
\repeatPhrasingSlurLeft?

Yikes.

As always, curious to know your thoughts.
- Mark