Hi Jan-Peter,
I know that this is a standard solution ; personnally, I'm not a big fan simply because this tie looks like... a tie ;)
So I'd suggest two options :
1. With \repeatTie plus a vertical tweak
2. With a hidden grace note.
So here it goes:
\version "2.18.2"
%% option 1:
{
% I want a slur from the b to the c in 1) and to the a in 2)
\repeat volta 2 { b'1 ( }
\alternative
{
{ c''1 ) }
{
a'1
-\tweak Y-offset #0.2 % <= for a better 'slur' look.
^\repeatTie
%% Description:
\tweak self-alignment-X #RIGHT
\mark\markup
\with-color #red
\fontsize #-2 {
Alternative slur with
\bold\typewriter\with-color #blue "\\repeatTie"
:
}
}
}
}
%% Option 2
{
% I want a slur from the b to the c in 1) and to the a in 2)
\repeat volta 2 { b'1 ( }
\alternative
{
{ c''1 ) }
{
\acciaccatura { % <= slured grace note
\once\slurUp % slur shows 'up'
\once\hideNotes b'1 % <= hide starting note
}
a'1
%% Description:
\tweak self-alignment-X #RIGHT
\mark\markup
\with-color #red
\fontsize #-2 { Here with a hidden grace note: }
}
}
}
Cheers,