boost::spirit, abstract syntax tree question

is there some trick to have an epsilon_p as a root_node?

(using ast_parse)

so id like this grammar to work:

Code:

expression_0
= expression_1 >> *(root_node_d[epsilon_p] >> expression_2) //!!! doesnt work
;

expression_1
= expression_2 >> *(
    (root_node_d[ch_p('+')] >> expression_2) // works perfectly
    (root_node_d[ch_p('-')] >> expression_2)
);

expression_2
= identifier
| value
| inner_node_d['(' >> expression_0 >> ')']
;


expression
= expression_0
;

unfortunately, there is no root node created for the rule expression_0.
so instead of building the abstract syntax tree for
"a b c d"
as

Code:

        []
       /  \d
      []
     /  \
   []    c
  /  \
a   b

the tree is built as:

Code:

       []
/    |     |     \
a   b    c      d

Did you post on boost spirit newsgroup?