code: def _check_paired(files, force_single, separators):
      
      full_name = _check_stems(files)
      if files[0].endswith(".bam"):
          return files
      elif is_gsm(files[0]):
          return files
      return combine_pairs(files, force_single, full_name, separators)
docs: check if files are fastq(.gz) and paired