code: def remove_root_metadata(docgraph):
      
      docgraph.node[docgraph.root].pop('metadata', None)
      # delete metadata from the generic root node (which probably only exists
      # when we merge graphs on the command line, cf. issue #89
      if 'discoursegraph:root_node' in docgraph.node:
          docgraph.node['discoursegraph:root_node'].pop('metadata', None)
      # delete the metadata from all former root nodes which have been merged
      # into this graph
      if hasattr(docgraph, 'merged_rootnodes'):
          for merged_rootnode in docgraph.merged_rootnodes:
              try:  # some of these nodes may not exist any longer
                  docgraph.node[merged_rootnode].pop('metadata', None)
              except KeyError as e:
                  pass
docs: removes the ``metadata`` attribute of the root node of a document graph.
      this is necessary for some exporters, as the attribute may contain
      (nested) dictionaries.