docs: def add_marker_to_qtl(qtl, map_list):
      """Add the closest marker to the given QTL.
  
      :arg qtl: a row of the QTL list.
      :arg map_list: the genetic map containing the list of markers.
  
      """
      closest = ''
      diff = None
      for marker in map_list:
          if qtl[1] == marker[1]:
              tmp_diff = float(qtl[2]) - float(marker[2])
              if diff is None or abs(diff) > abs(tmp_diff):
                  diff = tmp_diff
                  closest = marker
      if closest != '':
          closest = closest[0]
      return closest