template<typename target, typename subtarget, typename tag, typename... tags>
action<Back_back_deep_store_back<target, subtarget, tag, tags...>> struct
Convert and store value to vector of vector in state at position given by tags and target.
This struct and its apply function are used as a functor-like wrapper for calling the store_back member function of the underlying grammar stack. tag and tags... address a vector of vectors, whose inner value_type is a tagged tuple to which we store_back here after conversion, indexed by target and subtarget (hence deep).