code: def process_sub_shrink(ref, alt_str):
      
      if len(ref) == 0:
          raise exceptions.InvalidRecordException("Invalid VCF, empty REF")
      elif len(ref) == 1:
          if ref[0] == alt_str[0]:
              return record.Substitution(record.INS, alt_str)
          else:
              return record.Substitution(record.INDEL, alt_str)
      else:
          return record.Substitution(record.INDEL, alt_str)
docs: Process substution where the string shrink