code: def addChromosomeInstance(
              self, chr_num, reference_id, reference_label, chr_type=None):
          
          family = Family(self.graph)
          chr_id = makeChromID(str(chr_num), reference_id, 'MONARCH')
          chr_label = makeChromLabel(str(chr_num), reference_label)
  
          self.model.addIndividualToGraph(chr_id, chr_label, self.globaltt['chromosome'])
          if chr_type is not None:
              self.model.addType(chr_id, chr_type)
  
          # add the build-specific chromosome
          # as a member of the build  (both ways)
          family.addMember(reference_id, chr_id)
          family.addMemberOf(chr_id, reference_id)   # usage dependent, todo: ommit
  
          return
docs: Add the supplied chromosome as an instance within the given reference
          :param chr_num:
          :param reference_id: for example, a build id like UCSC:hg19
          :param reference_label:
          :param chr_type: this is the class that this is an instance of.
          typically a genome-specific chr
  
          :return: