convert SableCC grammar to ANTLR

I have the following very simple SableCC file:
Helpers

        letter = [ '_' + [['a' .. 'z'] + ['A' .. 'Z']]];
        digit  = ['0' .. '9'];
        cr     = 13;
        lf     = 10;
        tab    = 9;

Tokens

        lambda = 'lambda';
        ident  = letter (letter | digit)*;
        blank  = (9 | 10 | 13 | ' ')+;
        l_par  = '(';
        r_par  = ')';
        zero   = '0';
        one    = '1';
        plus   = '+';

Ignored Tokens
        
        blank;

Productions

        unit = exp+;
        
        exp  = {identifier} ident
             | {lambdaexpr} [first_l]:l_par lambda [second_l]:l_par ident*
[second_r]:r_par exp [first_r]:r_par
             | {exprseq}    l_par exp+ r_par
             | {zero}       zero
             | {one}        l_par plus exp one r_par;


And this is the translation to ANTLR I have so far:
class SubSchemeParser extends Parser;

options {
        buildAST=true;
}

unit : (exp)+;

exp  : IDENT
     | LPAREN "lambda" LPAREN IDENT* RPAREN exp RPAREN
     | LPAREN exp+ RPAREN
     | ZERO
     | LPAREN PLUS exp ONE RPAREN;


class SubSchemeLexer extends Lexer;

DIGIT : '0'..'9';
LPAREN : '(';
RPAREN : ')';
ZERO : '0';
ONE : '1';
PLUS : '+';

IDENT :    ID_START_LETTER ( ID_LETTER )*
    ;

protected
ID_START_LETTER
    :    '_'
    |    'a'..'z'
    |    'A'..'Z'
    ;

protected
ID_LETTER
    :    ID_START_LETTER
    |    '0'..'9'
    ;

// Whitespace -- ignored
WS
  : ( ' '
    | '\t'
    | '\f'

    // handle newlines
    | ( "\r\n" 
      | '\r'    
      | '\n'   
      )
      { newline(); }
    )
    { $setType(Token.SKIP); }
  ;

However on the seconde option of 'exp' I get these errors:
-rule classDef trapped
-unexpected token: IDENT

And I just cannot figure it out.
Is there anyone who can?
Thanks in advance.


PS if anyone is very good at ANTLR, maybe he/she could help get a good
translation. I need it for testing and comparison.