code: def population_variant_regions(items, merged=False):
      
      def _get_variant_regions(data):
          out = dd.get_variant_regions(data) or dd.get_sample_callable(data)
          # Only need to merge for variant region inputs, not callable BED regions which don't overlap
          if merged and dd.get_variant_regions(data):
              merged_out = dd.get_variant_regions_merged(data)
              if merged_out:
                  out = merged_out
              else:
                  out = merge_overlaps(out, data)
          return out
      import pybedtools
      if len(items) == 1:
          return _get_variant_regions(items[0])
      else:
          paired = vcfutils.get_paired(items)
          if paired:
              return _get_variant_regions(paired.tumor_data)
          else:
              vrs = []
              for data in items:
                  vr_bed = _get_variant_regions(data)
                  if vr_bed:
                      vrs.append((pybedtools.BedTool(vr_bed).total_coverage(), vr_bed))
              vrs.sort(reverse=True)
              if vrs:
                  return vrs[0][1]
docs: Retrieve the variant region BED file from a population of items.
  
      If tumor/normal, return the tumor BED file. If a population, return
      the BED file covering the most bases.