code: def add(self, namespace_uri):
          
          # See if this namespace is already mapped to an alias
          alias = self.namespace_to_alias.get(namespace_uri)
          if alias is not None:
              return alias
  
          # Fall back to generating a numerical alias
          i = 0
          while True:
              alias = 'ext' + str(i)
              try:
                  self.addAlias(namespace_uri, alias)
              except KeyError:
                  i += 1
              else:
                  return alias
  
          assert False, "Not reached"
docs: Add this namespace URI to the mapping, without caring what
          alias it ends up with