boost::spirit -- semantic values

I couldn't find a way to associate a semantic value with a
non-terminal. Bison/yacc allows this.

For example, here is a rule in Bison that says an expression can be
the sum of two subexpressions:

     expr: expr '+' expr   { $$ = $1 + $3; }
             ;

The action says how to produce the semantic value of the sum
expression from the values of the two subexpressions.

Having semantic values for non-terminals is useful to build an expression tree.

For, e.g, I can create a new node in the tree by passing in the
semantic values of child nodes in the action.
     expr: expr '+' expr   { $$ = makeNode($1, $3, "+"); }
             ;

In this case the semantic value for each expression is of type Node*.

Spirit provides parse trees and ASTs and also semantic actions that
take the first last iterator pair. But is there some way I can
associate a semantic value with each non-terminal  and pass in these
semantic values to the action for the rule.

Thanks,
Anand
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