code: def add_prefix(self, namespace, prefix):
          
          if prefix == "xml" and namespace != XML_NS:
              raise ValueError, "Cannot change 'xml' prefix meaning"
          self._prefixes[namespace] = prefix
docs: Add a new namespace prefix.
  
          If the root element has not yet been emitted the prefix will
          be declared there, otherwise the prefix will be declared on the
          top-most element using this namespace in every stanza.
  
          :Parameters:
              - `namespace`: the namespace URI
              - `prefix`: the prefix string
          :Types:
              - `namespace`: `unicode`
              - `prefix`: `unicode`