code: def by_regions(items):
      
      work_dir = os.path.join(dd.get_work_dir(items[0]), "structural", "coverage")
      safe_makedir(work_dir)
      out_file = os.path.join(work_dir, "%s-coverage.pdf" % (dd.get_sample_name(items[0])))
      if file_exists(out_file):
          items = _add_regional_coverage_plot(items, out_file)
      else:
          bed_files = _get_ensemble_bed_files(items)
          merged = bed.merge(bed_files)
          breakpoints = breakpoints_by_caller(bed_files)
          if merged:
              priority_merged = _prioritize_plot_regions(merged, items[0])
              out_file = plot_multiple_regions_coverage(items, out_file, items[0],
                                                        priority_merged, breakpoints)
              items = _add_regional_coverage_plot(items, out_file)
      return items
docs: Plot for a union set of combined ensemble regions across all of the data
         items.