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Changing boost::variant underlying type from visitor
struct sexpr {
      typedef boost::variant<
         nil,
         int,
         double,
         symbol,
         string,
         boost::recursive_wrapper<list<sexpr> >
      > node_type;

      node_type node;
   };

struct push_back_visitor: public boost::static_visitor<void>
   {
      push_back_visitor(const sexpr &arg): arg_(arg) {}

      template <typename T>
      void operator()(const T &value) const {
         throw bad_visit();
      }

      void operator()(nil &val) const {
         // how to change the underlying type to list<sexpr> here?
         // lst.push_back(arg_);
      }

      void operator()(list<sexpr> &lst) const {
         lst.push_back(arg_);
      }

      sexpr arg_;
   };

template<typename Variant, typename Visitor>
struct carry_variant_visitor
  : public boost::static_visitor<typename Visitor::result_type>
{
    carry_variant_visitor(Variant &variant, Visitor visitor):
        variant_(variant), visitor_(visitor) { }
    template<typename T>
    typename Visitor::result_type operator()(T &t) const {
        return visitor_(variant_, t);
    }
    Variant &variant_;
    Visitor visitor_;
}