extend SS chain using matching CA/CB shifts, and make sequence-specific assignments implied by beginning of SS chain
extend SS chain backwards:  3 SS matched the i-1 peaks of SS119:

 - SS76:  did not have a CB peak to match the i-1 of SS119
 - SS87:  the matching CA peak seemed to be an i-1, not an i
 - SS79:  this matched both the CA and CB peaks.  although it did
   not seem to have a CB(i-1) peak, this may just be overlapped