protected ATNConfigSet computeStartState(CharStream input,
											 ATNState p)
	{
		PredictionContext initialContext = EmptyPredictionContext.Instance;
		ATNConfigSet configs = new OrderedATNConfigSet();
		for (int i=0; i<p.getNumberOfTransitions(); i++) {
			ATNState target = p.transition(i).target;
			LexerATNConfig c = new LexerATNConfig(target, i+1, initialContext);
			closure(input, c, configs, false, false, false);
		}
		return configs;
	}
Given a starting configuration set, figure out all ATN configurations
 we can reach upon input {@code t}. Parameter {@code reach} is a return
 parameter.
runtime/Java/src/org/antlr/v4/runtime/atn/LexerATNSimulator.java