Write a namespace dictionary, taking care to not clobber the
        standard (or reserved by us) prefixes.

def writeNSdict(self, nsdict):
        '''Write a namespace dictionary, taking care to not clobber the
        standard (or reserved by us) prefixes.
        '''
        for k,v in nsdict.items():
            if (k,v) in _standard_ns: continue
            rv = _reserved_ns.get(k)
            if rv:
                if rv != v:
                    raise KeyError("Reserved namespace " + str((k,v)) + " used")
                continue
            if k:
                self.dom.setNamespaceAttribute(k, v)
            else:
                self.dom.setNamespaceAttribute('xmlns', v)