docs: def add(self, r):
          '''Takes an id and a Residue r and adds them to the Sequence.'''
          id = r.get_residue_id()
          if self.order:
              last_id = self.order[-1]
  
              # KAB - allow for multiresidue noncanonicals
              if id in self.order:
                  raise colortext.Exception('Warning: using code to "allow for multiresidue noncanonicals" - check this case manually.')
                  id = '%s.%d'%(str(id),self.special_insertion_count)
                  self.special_insertion_count += 1
  
              assert(r.Chain == self.sequence[last_id].Chain)
              assert(r.residue_type == self.sequence[last_id].residue_type)
          self.order.append(id)
          self.sequence[id] = r