public IntervalSet getExpectedTokens(int stateNumber, RuleContext context) {
		if (stateNumber < 0 || stateNumber >= states.size()) {
			throw new IllegalArgumentException("Invalid state number.");
		}

		RuleContext ctx = context;
		ATNState s = states.get(stateNumber);
		IntervalSet following = nextTokens(s);
		if (!following.contains(Token.EPSILON)) {
			return following;
		}

		IntervalSet expected = new IntervalSet();
		expected.addAll(following);
		expected.remove(Token.EPSILON);
		while (ctx != null && ctx.invokingState >= 0 && following.contains(Token.EPSILON)) {
			ATNState invokingState = states.get(ctx.invokingState);
			RuleTransition rt = (RuleTransition)invokingState.transition(0);
			following = nextTokens(rt.followState);
			expected.addAll(following);
			expected.remove(Token.EPSILON);
			ctx = ctx.parent;
		}

		if (following.contains(Token.EPSILON)) {
			expected.add(Token.EOF);
		}

		return expected;
	}
Computes the set of input symbols which could follow ATN state number
{@code stateNumber} in the specified full {@code context}. This method
considers the complete parser context, but does not evaluate semantic
predicates (i.e. all predicates encountered during the calculation are
assumed true). If a path in the ATN exists from the starting state to the
{@link RuleStopState} of the outermost context without matching any
symbols, {@link Token#EOF} is added to the returned set.

<p>If {@code context} is {@code null}, it is treated as {@link ParserRuleContext#EMPTY}.</p>

Note that this does NOT give you the set of all tokens that could
appear at a given token position in the input phrase.  In other words,
it does not answer:

  "Given a specific partial input phrase, return the set of all tokens
   that can follow the last token in the input phrase."

The big difference is that with just the input, the parser could
land right in the middle of a lookahead decision. Getting
all *possible* tokens given a partial input stream is a separate
computation. See https://github.com/antlr/antlr4/issues/1428

For this function, we are specifying an ATN state and call stack to compute
what token(s) can come next and specifically: outside of a lookahead decision.
That is what you want for error reporting and recovery upon parse error.

@param stateNumber the ATN state number
@param context the full parse context
@return The set of potentially valid input symbols which could follow the
specified state in the specified context.
@throws IllegalArgumentException if the ATN does not contain a state with
number {@code stateNumber}
runtime/Java/src/org/antlr/v4/runtime/atn/ATN.java