docs: def plot_spectra_pieces_pdf(ss, aint=10, pdf_filename='pieces.pdf', setup=_default_setup):
      """
      Plots spectra, overlapped, in small wavelength intervals into a PDF file,
      one interval per page of the PDF file.
  
      Args:
        ss: list of Spectrum objects
        aint: wavelength interval for each plot
        pdf_filename: name of output file
        setup: PlotSpectrumSetup object
  
      **Note** overrides setup.fmt_xlabel; leaves y-labell and title blank
      """
  
      import f311.explorer as ex
  
      xmin, xmax, ymin_, ymax, _, yspan = calc_max_min(ss)
      ymin = ymin_ if setup.ymin is None else setup.ymin
  
      num_pages = int(math.ceil((xmax-xmin)/aint)) # rightmost point may be left out...or not
      # num_spectra = len(ss)
  
      a99.format_BLB()
      # pdf = matplotlib.backends.backend_pdf.PdfPages(pdf_filename)
      pdf = matplotlib.backends.backend_pdf.PdfPages(pdf_filename)
      logger = a99.get_python_logger()
  
      for h in range(num_pages):
          fig = plt.figure()
          lambda0 = xmin+h*aint
          lambda1 = lambda0+aint
          logger.info("Printing page {0:d}/{1:d} ([{2:g}, {3:g}])".format(h+1, num_pages, lambda0, lambda1))
          for i, s in enumerate(ss):
              s_cut = ex.cut_spectrum(s, lambda0, lambda1)
              ax = plt.gca()
              ax.plot(s_cut.x, s_cut.y, label=s.title)
          if setup.flag_xlabel and setup.fmt_xlabel:
              plt.xlabel('Wavelength (interval: [{0:g}, {1:g}])'.format(lambda0, lambda1))
          xspan = lambda1-lambda0
          ax.set_xlim([lambda0 - xspan * _T, lambda1 + xspan * _T])
          ax.set_ylim([ymin - yspan * _T, ymax + yspan * _T])
          if setup.flag_legend:
              leg = plt.legend(loc=0)
              a99.format_legend(leg)
          plt.tight_layout()
          pdf.savefig(fig)
          plt.close()
  
      # for fig in xrange(1, figure().number): ## will open an empty extra figure :(
      #     pdf.savefig( fig )
      pdf.close()
      logger.info("File {0!s} successfully created.".format(pdf_filename))