code: def update_clr(self, aclr, bclr):
          
          print(aclr, bclr, file=sys.stderr)
          otype = self.otype
  
          if otype == 1:
              if aclr.orientation == '+':
                  aclr.end = self.qstop
              else:
                  aclr.start = self.qstart
              if bclr.orientation == '+':
                  bclr.start = self.sstop + 1
              else:
                  bclr.end = self.sstart - 1
  
          elif otype == 3:
              aclr.start = aclr.end
  
          elif otype == 4:
              bclr.start = bclr.end
  
          print(aclr, bclr, file=sys.stderr)
docs: Zip the two sequences together, using "left-greedy" rule
  
          =============                   seqA
                   ||||
                   ====(===============)  seqB