@Override
	public Handle action(String action) {
		throw new UnsupportedOperationException("This element is not valid in parsers.");
	}
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