func (w *http2responseWriter) Push(target string, opts *PushOptions) error {
	st := w.rws.stream
	sc := st.sc
	sc.serveG.checkNotOn()

	// No recursive pushes: "PUSH_PROMISE frames MUST only be sent on a peer-initiated stream."
	// http://tools.ietf.org/html/rfc7540#section-6.6
	if st.isPushed() {
		return http2ErrRecursivePush
	}

	if opts == nil {
		opts = new(PushOptions)
	}

	// Default options.