docs: def _merge_region_trees(self, dst_tree, src_tree, pid):
          """Merge conflicts occur if a folder in one tree is a file in the other.
  
          As the files are PIDs, this can only happen if a PID matches one of the
          geographical areas that the dataset covers and should be very rare. In such
          conflicts, the destination wins.
  
          """
          for k, v in list(src_tree.items()):
              # Prepend an underscore to the administrative area names, to make them
              # sort separately from the identifiers.
              # k = '_' + k
              if k not in dst_tree or dst_tree[k] is None:
                  dst_tree[k] = {}
              dst_tree[k][pid] = None
              if v is not None:
                  self._merge_region_trees(dst_tree[k], v, pid)