code: def index(in_cram, config):
      
      out_file = in_cram + ".crai"
      if not utils.file_uptodate(out_file, in_cram):
          with file_transaction(config, in_cram + ".crai") as tx_out_file:
              tx_in_file = os.path.splitext(tx_out_file)[0]
              utils.symlink_plus(in_cram, tx_in_file)
              cmd = "samtools index {tx_in_file}"
              do.run(cmd.format(**locals()), "Index CRAM file")
      return out_file
docs: Ensure CRAM file has a .crai index file.