protected Token getMissingSymbol(Parser recognizer) {
		Token currentSymbol = recognizer.getCurrentToken();
		IntervalSet expecting = getExpectedTokens(recognizer);
		int expectedTokenType = Token.INVALID_TYPE;
		if ( !expecting.isNil() ) {
			expectedTokenType = expecting.getMinElement(); // get any element
		}
		String tokenText;
		if ( expectedTokenType== Token.EOF ) tokenText = "<missing EOF>";
		else tokenText = "<missing "+recognizer.getVocabulary().getDisplayName(expectedTokenType)+">";
		Token current = currentSymbol;
		Token lookback = recognizer.getInputStream().LT(-1);
		if ( current.getType() == Token.EOF && lookback!=null ) {
			current = lookback;
		}
		return
			recognizer.getTokenFactory().create(new Pair<TokenSource, CharStream>(current.getTokenSource(), current.getTokenSource().getInputStream()), expectedTokenType, tokenText,
							Token.DEFAULT_CHANNEL,
							-1, -1,
							current.getLine(), current.getCharPositionInLine());
	}
Conjure up a missing token during error recovery.

 The recognizer attempts to recover from single missing
 symbols. But, actions might refer to that missing symbol.
 For example, x=ID {f($x);}. The action clearly assumes
 that there has been an identifier matched previously and that
 $x points at that token. If that token is missing, but
 the next token in the stream is what we want we assume that
 this token is missing and we keep going. Because we
 have to return some token to replace the missing token,
 we have to conjure one up. This method gives the user control
 over the tokens returned for missing tokens. Mostly,
 you will want to create something special for identifier
 tokens. For literals such as '{' and ',', the default
 action in the parser or tree parser works. It simply creates
 a CommonToken of the appropriate type. The text will be the token.
 If you change what tokens must be created by the lexer,
 override this method to create the appropriate tokens.
runtime/Java/src/org/antlr/v4/runtime/DefaultErrorStrategy.java