old_contents: from __future__ import absolute_import
  from .twobit import TwoBitFile
  
  
  def get_reference_allele(chrom, start, hg19_path):
      twobit_file = TwoBitFile(hg19_path)
      end = start + 1
      refallele = twobit_file[chrom][start:end]
      return refallele
  
new_contents: from __future__ import absolute_import
  from .twobit import TwoBitFile
  
  
  def get_reference_allele(chrom, start, hg19_path):
      twobit_file = TwoBitFile(hg19_path)
      end = start + 1
      try:
          refallele = twobit_file[chrom][start:end]
      except TypeError:
          refallele = 'N'
      return refallele
  
subject: Return N for ref allele in pos uncovered by ref