def envelope(self, header, body):
        """
        Build the B{<Envelope/>} for a SOAP outbound message.

        @param header: The SOAP message B{header}.
        @type header: L{Element}
        @param body: The SOAP message B{body}.
        @type body: L{Element}
        @return: The SOAP envelope containing the body and header.
        @rtype: L{Element}

        """
        env = Element("Envelope", ns=envns)
        env.addPrefix(Namespace.xsins[0], Namespace.xsins[1])
        env.append(header)
        env.append(body)
        return env