protected void epsilon(ATNState a, ATNState b, boolean prepend) {
		if ( a!=null ) {
			int index = prepend ? 0 : a.getNumberOfTransitions();
			a.addTransition(index, new EpsilonTransition(b));
		}
	}
Build an atom with all possible values in its label.
tool/src/org/antlr/v4/automata/ParserATNFactory.java