Slurs with "afterGrace"

[Alasdair McAndrew]

This works:
g4( \grace {a16 g16})

making a slur which includes the grace notes.  But in order to get the 
right spacing and barring (I need grace notes to occur before the bar line, 
rather than after), I need to use "afterGrace":

\afterGrace g4 {a16 g16}

However, if I attempt to include a slur:

\afterGrace g4( {a16 g16} )

I get a warning about an "Unattached SlurEvent".  Lilypond makes a good 
guess at what I want and bungs a slur in anyway - but what does this 
warning mean, and how can I avoid it?  Thank you!

Alasdair
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