code: def siblings_before(self):
          
          impl_nodelist = self.adapter.get_node_children(self.parent.impl_node)
          before_nodelist = []
          for n in impl_nodelist:
              if n == self.impl_node:
                  break
              before_nodelist.append(n)
          return self._convert_nodelist(before_nodelist)
docs: :return: a list of this node's siblings that occur *before* this
              node in the DOM.