ANTLR4 grammar. parses and returns int expression e.g. "-2 * --(1+-1)"

gistfile1.java

grammar negationtest;

expression: ar_expression { System.out.println($ar_expression.ret); }
          ;

ar_expression returns [Integer ret]:
                 a = mult_expression { $ret = $a.ret; }
                 (
                    ('+' b = mult_expression { $ret += $b.ret; } ) |
                    ('-' c = mult_expression { $ret -= $c.ret; } )
                 )*
             ;

mult_expression returns [Integer ret]:
                   a = atom { $ret = $a.ret; } (
                            ('*' b = atom { $ret *= $b.ret; } ) | 
                            ('/' c = atom { $ret /= $c.ret; } )
                        )*
               ;

atom returns [Integer ret]:
        ('-' a = atom {$ret = - $a.ret; } ) |
        b = atom_real { $ret = $b.ret; }
    ;

atom_real returns [Integer ret]:
        INT {$ret = Integer.parseInt($INT.text); } | 
        '(' ar_expression { $ret = $ar_expression.ret; } ')'
         ;


INT : '0'..'9'+ ;
ID  : 'a'..'z'+ ;
WS : (' '|'\r'|'\n') -> skip;