I'm looking for replacements for the events
onBeforeAddToCart to prevent the user from adding an article from different article groups to the cart.
onBeforeGatewayRequestSend is used to check a mandatory field and to increment our own order/invoice numbers (the md5-style numbers are not convenient for German business).
I need to set
$event->performAction = false and
$transaction->message = "Please agree with our terms and conditions" if the mandatory field is not checked, like I did in Commerce 1.