[antlr-interest] predicate, uh?

[Lloyd Dupont]

I have a simple grammar rule like that (inheriting from cgram)

objcMethodDefinition  returns [String selName]
{
 selName = "";
}
 : ( PLUS | MINUS ) (castExpr)? 
  (
    ( ID COLON )=> 
     id0:ID COLON (castExpr)? ID { selName = id0.getText() + ":"; }
     ( 
      (id1:ID  { selName += id1.getText(); })? (COLON) (castExpr)? ID { selName += ":"; }
     ) *
   | id2:ID { selName = id2.getText(); }
  )
 
note: there is no explicit end to this rule, because I reuse it for both declaration (ending by a ';') and definition (ending by '{').

when I test it I get this parsing error (while evaluating the (ID COLON)=> !!!) (on something as simple as "- foo { ...."
ANTLR Parsing Error: # 1 "":line 22:1896: unexpected token: { token name:LCURLY
line 22:1896: unexpected token: {
        at ObjectiveCParser.postfixExpr(ObjectiveCParser.java:5876)
        at ObjectiveCParser.unaryExpr(ObjectiveCParser.java:5293)
        at ObjectiveCParser.castExpr(ObjectiveCParser.java:1548)
        at ObjectiveCParser.objcMethodDefinition(ObjectiveCParser.java:1129)
        at ObjectiveCParser.objcClassImplementation(ObjectiveCParser.java:1056)
        at ObjectiveCParser.externalDef(ObjectiveCParser.java:213)
        at ObjectiveCParser.externalList(ObjectiveCParser.java:1649)
        at ObjectiveCParser.translationUnit(ObjectiveCParser.java:1610)
        at Test.main(Test.java:40)

?!!!
how come? 
what did I do wrong?
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