protected boolean checkRange(GrammarAST leftNode, GrammarAST rightNode, int leftValue, int rightValue) {
		boolean result = true;
		if (leftValue == -1) {
			result = false;
			g.tool.errMgr.grammarError(ErrorType.INVALID_LITERAL_IN_LEXER_SET,
					g.fileName, leftNode.getToken(), leftNode.getText());
		}
		if (rightValue == -1) {
			result = false;
			g.tool.errMgr.grammarError(ErrorType.INVALID_LITERAL_IN_LEXER_SET,
					g.fileName, rightNode.getToken(), rightNode.getText());
		}
		if (!result) return false;

		if (rightValue < leftValue) {
			g.tool.errMgr.grammarError(ErrorType.EMPTY_STRINGS_AND_SETS_NOT_ALLOWED,
					g.fileName, leftNode.parent.getToken(), leftNode.getText() + ".." + rightNode.getText());
			return false;
		}
		return true;
	}
Maps from a {@link LexerAction} object to the action index.
tool/src/org/antlr/v4/automata/LexerATNFactory.java