@Override
	public Handle action(ActionAST action) {
		//System.out.println("action: "+action);
		ATNState left = newState(action);
		ATNState right = newState(action);
		ActionTransition a = new ActionTransition(right, currentRule.index);
		left.addTransition(a);
		action.atnState = left;
		return new Handle(left, right);
	}
Build what amounts to an epsilon transition with an action.
 The action goes into ATN though it is ignored during prediction
 if {@link ActionTransition#actionIndex actionIndex}{@code <0}.
tool/src/org/antlr/v4/automata/ParserATNFactory.java