[antlr-interest] Stuck with parser & tree grammar

[G R]

Hi all!
I'm still trying to translate relational algebra into SQL.
I used to some problems with my grammar, so I decide to change it into a
LL(1) grammar.
So, a relational algebra query can be describe like this :

raExpr
:    raQuery^
|    RelationName^;

raQuery
:    project^
|    select^
|    rename^
|    binaryExpr^;

binaryExpr
:    r1=relation (BinaryOp r2=relation
|     LeftBrack a1=AttributeName RightBrack Join r2=relation LeftBrack
a2=AttributeName RightBrack)
    ->{$BinaryOp!=null}?^(BinaryOp $r1 $r2)
    ->^(Join ^(Left $r1 $a1) ^(Right $r2 $a2));

relation
:    LeftParent raQuery RightParent
    ->^(raQuery)
|    RelationName
    ->^(RelationName);

As you can see, I'm trying to make possible input like :
relationName1 UNION relationName2
Or
relationName1[AttributeName1] JOIN relationName2[AttributeName2]

Using this grammar, this is ok : my AST looks good. Using previous inputs, I
got the following AST :
(UNION relationName1 relationName2)
Or
(Join (Left relationName1, AttributeName1) (Right relationName2,
AttributeName2))

The problem is in the tree grammar. I don't know how to deal with the rule
binaryExpr.
I've try this :
binaryExpr
:    ^(BinaryOp r1=relation r2=relation)
        -> {BinaryOp==Union}? Union(left={$r1.st}, right={$r2.st})
        -> {BinaryOp==Intersect}? Intersect(left={$r1.st}, right={$r2.st})
        -> {BinaryOp==Difference}? Difference(left={$r1.st}, right={$r2.st})
        -> {BinaryOp==Divide}? Divide(left={$r1.st}, right={$r2.st})
|    ^(Join ^(Left r1=relation a1=AttributeName) ^(Right r2=relation
a2=AttributeName))
        -> join(leftRel={$lr.st}, leftAtt={$la.text}, leftAlias={$lr.alias},
                        rightRel={$rr.st}, rightAtt={$ra.text},
rightAlias={$rr.alias});

But, ANTLR is expecting a REWRITE instead of the optional operator ( | )
just after the Divide.

So, my question is : how can I generate my walker using the previous parser
grammar, or What should I do for this to work ?

Thanks for any help.

G.R.
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