[antlr-interest] Please help to left-refactor this simple ebnf grammar

[Artem Golubev]

I just can't get how I can left-refactor the following grammar. Please,
help.
Currently I get "(211): ebnf.g:1:8: [fatal] rule Tokens has non-LL(*)
decision due to recursive rule invocations reachable from alts 8,9. Resolve
by left-factoring or using syntactic predicates or using backtrack=true
option." error with it.

grammar ebnf;

options {
output = AST;
language = CSharp2;
}

tokens {
OPENBRACKET = '(';
CLOSEBRACKET = ')';
ONEORMOREKEYWORD= '+';
ANYKEYWORD = '*';
OPTIONALKEYWORD = '?';
ORKEYWORD = '|';
}

/*------------------------------------------------------------------
* PARSER RULES
*------------------------------------------------------------------*/
OrList : List (ORKEYWORD List)*;

List : (Mutliplication)+;

Mutliplication : Atom ( ONEORMOREKEYWORD | OPTIONALKEYWORD | ANYKEYWORD )?;

Atom : IDENTIFIER | Group;

Group : OPENBRACKET OrList CLOSEBRACKET;

/*------------------------------------------------------------------
* LEXER RULES
*------------------------------------------------------------------*/

IDENTIFIER : (DIGIT)+ ;

WHITESPACE : ( '\t' | ' ' | '\r' | '\n' )+ { $channel = HIDDEN; } ;

fragment DIGIT : '0'..'9' ;
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