code: def startPrefixMapping(self, prefix, uri, *, auto=False):
          
          if     (prefix is not None and
                  (prefix == "xml" or
                   prefix == "xmlns" or
                   not xmlValidateNameValue_str(prefix) or
                   ":" in prefix)):
              raise ValueError("not a valid prefix: {!r}".format(prefix))
  
          if prefix in self._ns_prefixes_floating_in:
              raise ValueError("prefix already declared for next element")
          if auto:
              self._ns_auto_prefixes_floating_in.add(prefix)
          self._ns_prefixes_floating_in[prefix] = uri
          self._ns_decls_floating_in[uri] = prefix
docs: Start a prefix mapping which maps the given `prefix` to the given
          `uri`.
  
          Note that prefix mappings are handled transactional. All announcements
          of prefix mappings are collected until the next call to
          :meth:`startElementNS`. At that point, the mappings are collected and
          start to override the previously declared mappings until the
          corresponding :meth:`endElementNS` call.
  
          Also note that calling :meth:`startPrefixMapping` is not mandatory; you
          can use any namespace you like at any time. If you use a namespace
          whose URI has not been associated with a prefix yet, a free prefix will
          automatically be chosen. To avoid unneccessary performance penalties,
          do not use prefixes of the form ``"ns{:d}".format(n)``, for any
          non-negative number of `n`.
  
          It is however required to call :meth:`endPrefixMapping` after a
          :meth:`endElementNS` call for all namespaces which have been announced
          directly before the :meth:`startElementNS` call (except for those which
          have been chosen automatically). Not doing so will result in a
          :class:`RuntimeError` at the next :meth:`startElementNS` or
          :meth:`endElementNS` call.
  
          During a transaction, it is not allowed to declare the same prefix
          multiple times.