magarena/release/Magarena/scripts/Mystic_Snake.groovy

[
    new MagicWhenComesIntoPlayTrigger() {
        @Override
        public MagicEvent executeTrigger(final MagicGame game,final MagicPermanent permanent, final MagicPlayer player) {
            return new MagicEvent(
                permanent,
                player,
                MagicTargetChoice.TARGET_SPELL,
                this,
                "Counter target spell\$."
            );
        }
        @Override
        public void executeEvent(final MagicGame game, final MagicEvent event) {
            event.processTargetCardOnStack(game,new MagicCardOnStackAction() {
                public void doAction(final MagicCardOnStack targetSpell) {
                    game.doAction(new MagicCounterItemOnStackAction(targetSpell));
                }
            });
        }
    }
]