code: def read_fastas(input_files):
      
      tumor_file = [y for x, y in input_files.items() if x.startswith('T')][0]
      normal_file = [y for x, y in input_files.items() if x.startswith('N')][0]
      output_files = defaultdict(list)
      output_files = _read_fasta(tumor_file, output_files)
      num_entries = len(output_files)
      output_files = _read_fasta(normal_file, output_files)
      assert len(output_files) == num_entries
      return output_files
docs: Read the tumor and normal fastas into a joint dict.
  
      :param dict input_files: A dict containing filename: filepath for T_ and N_ transgened files.
      :return: The read fastas in a dictionary of tuples
      :rtype: dict