code: def get_connective(docgraph, unit_id):
      
      unit_index, _unit_type = unit_id.split(':')
      connective_id = unit_index+':connective'
      return ' '.join(docgraph.get_token(tok_id).lower()
                      for tok_id in get_span(docgraph, connective_id))
docs: returns the lowercased string of the connective used in the given Conano unit.