identify sequential SS chain and make i/i-1 peak assignments,
based on matching CA/CB shifts

(no sequence-specific assignment)
make short sequential SS chain, sequence-specific and i/i-1 assignment,
based on:

 - CA/CB shifts for sequential assignment
 - clearly had an A followed by a G, of which there was only one
   left unassigned in the sequence
extend sequential SS chain backwards, based on matching CA/CB shifts.  there is a lot of overlap of i/i-1 peaks, and SS9 has extremely weak i peaks, and SS80 has weaker i peaks than i-1, but the data are consistent with matching strip plots and with the sequence