[antlr-interest] SimpleCalc Tree Rewrite Rules

[Matthew Bowman]

So I have the following grammar:

grammar SimpleCalcTree;
options { output=AST; }
tokens{ OP; NUM; }

expr : l=term (o=plus_minus r=term)* -> ^(OP $o $l $r);
term : l=factor (o=mult_divide r=factor)* -> ^(OP $o $l $r);
factor : NUMBER -> ^(NUM NUMBER);
plus_minus : PLUS | MINUS;
mult_divide : MULT | DIVIDE;

PLUS : '+';
MINUS : '-';
MULT : '*';
DIVIDE : '/';
NUMBER : '0'..'9'+;

And my problem is with the rewrite rules. I want it to only rewrite the 
rule with (OP $o $l $r) if it actually matches 'term plus_minus term' or 
'factor mult_divide factor' but if it only matches a term or factor I 
want the subtree from factor.

Examples of what I'm trying to accomplish (and support order of 
operations)...

Input: 2
Output: (NUM 2)

Input: 2 + 2
Output: (OP + (NUM 2) (NUM 2))

Input: 2 * 2 - 4
Output: (OP - (OP * (NUM 2) (NUM 2)) (NUM 4))

My real world example is much more complex but this if I can solve this 
trivial case then I can adapt it to what I need.

Thanks in advance!

-- 
Matthew Bowman