I am try to undestand the  calc2_ast.cpp example at http://www.boost.org/doc/libs/1_41_0/libs/spirit/example/qi/calc2_ast.cpp
I think the instruction expression_ast() : expr(nil()){} is an inheritance. Where can I find the definition of expr()?
This is the code
 struct expression_ast
    {
        typedef
            boost::variant<
                nil // can't happen!
              , unsigned int
              , boost::recursive_wrapper<expression_ast>
              , boost::recursive_wrapper<binary_op>
              , boost::recursive_wrapper<unary_op>
            >
        type;

        expression_ast() : expr(nil()){};

        template <typename Expr>
        expression_ast(Expr const& expr)
          : expr(expr) {}

        expression_ast& operator+=(expression_ast const& rhs);
        expression_ast& operator-=(expression_ast const& rhs);
        expression_ast& operator*=(expression_ast const& rhs);
        expression_ast& operator/=(expression_ast const& rhs);

        type expr;
    };

    //expression_ast bbb;
    struct binary_op
    {
        binary_op(
            char op
          , expression_ast const& left
          , expression_ast const& right)
        : op(op), left(left), right(right) {}

        char op;
        expression_ast left;
        expression_ast right;
    };