public IFieldReader <TItem> Get(Stem <TItem> stem)
        {
            var substemCreator = new SubstemEngineSubContextCreator <TItem, TNav, TSubstem>(stem);
            StemsEngineSubContext <TNav> subContext = substemCreator.GetEngineContext();

            return(new SubCollectionFieldReader <TItem, TCollection, TNav>(_navigationExpression, subContext));
        }
        public IFieldWriter <TItem> Get(Stem <TItem> stem)
        {
            var substemCreator = new SubstemEngineSubContextCreator <TItem, TNav, TSubstem>(stem);
            StemDataChangeEvents <TNav>  stemEvents = substemCreator.GetDataChangeEvents();
            StemsEngineSubContext <TNav> subContext = substemCreator.GetEngineContext();

            MethodSetter <TItem, TNav> setter = MethodSetter <TItem, TNav> .FromDefinition(_definition, stem);

            var tools = new SubWriterTools <TItem, TNav, TNav>(_navigationExpression, stemEvents, setter);

            return(new SubItemFieldWriter <TItem, TNav>(tools, subContext));
        }