SAR well tested?

Hello,
I started to look into the kannel code a few days ago (very few) 
and I am currently following the SAR implementation, finding a
few things that need clarification (beside being a newbie on
kannel I also happen to be a newbie on WAP).
However, here is the question regarding the WTP responder.
On reception of a SARed transaction, process_sar_transaction()
adds the first invoke and the following segm-invokes to the
list of received blocks using add_sar_transaction().
When it is time to ACK/NAK a group (GTR=1), there is no check for a
complete reception of all blocks in a group, and it is always
built an ACK packet despite the fact that one of them may have been 
lost. 
Sending a segmented result does not seem to suffer of the same 
problem as the state machine is prepared to receive a NAK and to
resent the lost blocks.
I am sure I am missing something and hope that someone can help
shed some light.
Thanks
Marco