docs: function(){
          return {
            classes: [], 
            colonSelectors: [],
            data: [],
            group: null,
            ids: [],
            meta: [],
  
            // fake selectors
            collection: null, // a collection to match against
            filter: null, // filter function
  
            // these are defined in the upward direction rather than down (e.g. child)
            // because we need to go up in Selector.filter()
            parent: null, // parent query obj
            ancestor: null, // ancestor query obj
            subject: null, // defines subject in compound query (subject query obj; points to self if subject)
  
            // use these only when subject has been defined
            child: null,
            descendant: null
          };
        }