stem length-grace note

[Stan Sanderson]

I'm using lily v2.2.0 and have a problem which I've not been able to 
solve.

A two-note acciaccatura precedes an arpeggio chord. The piece is in 3/4 
time. The code for the measure is as follows:

r4 \acciaccatura {af,16[ af']} <af c gf'>8\arpeggio r8 r4

The problem is that the upper af of the acciaccatura is covered by the 
bar. I would like to lengthen the stems of the acciaccatura but 
obviously don't understand what I am doing! Help would be much 
appreciated.

Thanks,

Stan