def setPrefix(self, p, u=None):
        """
        Set the element namespace prefix.
        @param p: A new prefix for the element.
        @type p: basestring
        @param u: A namespace URI to be mapped to the prefix.
        @type u: basestring
        @return: self
        @rtype: L{Element}
        """
        self.prefix = p
        if p is not None and u is not None:
            self.addPrefix(p, u)
        return self