FBChangeDetailsBegin() with FBStoryTrack

Hi,

Does anyone know how to use the FBChangeDetailsBegin() and FBChangeDetailsEnd()
functions when creating a story track?  The docs mention
'You must call this function before adding/removing any object to the Details
list or it won't work.'
but I can't find a method to actually change the FBStoryTrack.Details tuple. 
.append isn't recognised, and a wholescale replacement of the list doesn't work
either.

I know I've tried this in the past with no success, but am revisiting the
problem and can't remember trying the FBChangeDetails methods.

Thanks!