code: def promotePrefixes(self):
          
          for c in self.children:
              c.promotePrefixes()
          if self.parent is None:
              return
          for p, u in self.nsprefixes.items():
              if p in self.parent.nsprefixes:
                  pu = self.parent.nsprefixes[p]
                  if pu == u:
                      del self.nsprefixes[p]
                  continue
              if p != self.parent.prefix:
                  self.parent.nsprefixes[p] = u
                  del self.nsprefixes[p]
          return self
docs: Push prefix declarations up the tree as far as possible.
  
          Prefix mapping are pushed to its parent unless the parent has the
          prefix mapped to another URI or the parent has the prefix. This is
          propagated up the tree until the top is reached.
  
          @return: self
          @rtype: L{Element}