code: def add_marker_to_qtls(qtlfile, mapfile, outputfile='qtls_with_mk.csv'):
      
      qtl_list = read_input_file(qtlfile, ',')
      map_list = read_input_file(mapfile, ',')
      if not qtl_list or not map_list:  # pragma: no cover
          return
      qtl_list[0].append('Closest marker')
      qtls = []
      qtls.append(qtl_list[0])
      for qtl in qtl_list[1:]:
          qtl.append(add_marker_to_qtl(qtl, map_list))
          qtls.append(qtl)
      LOG.info('- %s QTLs processed in %s' % (len(qtls), qtlfile))
      write_matrix(outputfile, qtls)
docs: This function adds to a list of QTLs, the closest marker to the
      QTL peak.
  
      :arg qtlfile: a CSV list of all the QTLs found.
          The file should be structured as follow::
              Trait, Linkage group, position, other columns
  
          The other columns will not matter as long as the first three
          columns are as such.
      :arg mapfile: a CSV representation of the map used for the QTL
          mapping analysis.
          The file should be structured as follow::
              Marker, Linkage group, position
      :kwarg outputfile: the name of the output file in which the list of
          QTLs with their closest marker will be written.