\afterGrace control over stem length

[James Harkins]

Hi, I have a couple of \afterGrace's in close proximity. One has normal-looking 
stems on the grace notes (attached, first bar). The other's stems are too long 
(third bar).

I'm not tweaking, overriding or setting anything at that point in the input 
code:

  g4 ~ \> g8 ) af4. ~ (
  af g8 ~ g4 ) r8 \p \afterGrace c8 ( \mp\< { d16 [ e ] }
  f8 \> e ) d16 ( c8. ) \! f,4. r8
  r2 r4 \clef bass \afterGrace d4 \mp _( { df16 [ c ] }
  b4. ) \< g8 ~ g16 f8. c4-> ~ \f\>

(And, just for fun, I tried moving "\clef bass" before the r4 -- no effect, as 
I would have expected.)

I'm satisfied to override the stem length, but I'm not finding the answer in 
the manual. Stem overrides seem to be for unbeamed stems. Beam overrides are... 
opaque to say the least.

How do I shorten these stems by 1 or 2 staff spaces? (I know the override needs 
to come inside the { df16 ... } expression.)

Thanks.

hjh