code: func (h *Host) serveImporterAcctCallback(w http.ResponseWriter, r *http.Request, imp *importer) {
  	if r.Method != "GET" {
  		http.Error(w, "invalid method", 400)
  		return
  	}
  	acctRef, err := imp.impl.CallbackRequestAccount(r)
  	if err != nil {
  		httputil.ServeError(w, r, err)
  		return
  	}
  	if !acctRef.Valid() {
  		httputil.ServeError(w, r, errors.New("No valid blobref returned from CallbackRequestAccount(r)"))
  		return
  	}
  	ia, err := imp.account(acctRef)
  	if err != nil {
  		http.Error(w, "invalid 'acct' param: "+err.Error(), 400)
  		return
  	}
  	imp.impl.ServeCallback(w, r, &SetupContext{
  		Context:     context.TODO(),
  		Host:        h,
  		AccountNode: ia.acct,
  		ia:          ia,
  	})
  }
docs: // Serves oauth callback at http://host/importer/TYPE/callback