extend SS chain backwards:  three SS matched the i-1 peaks of SS79:

 - SS77's matching peaks seemed to be i-1 peaks, and it was not preceded
   by a Glycine as demanded by the sequence -- since SS79 was already
   assigned to a specific residue
 - SS51 was not preceded by a Glycine
 - SS110 was preceded by a Glycine, and its i peaks matched the i-1
   peaks of ss79