code: function(local, available) {
          let updates = [];
          for(let a of available) {
              try {
                  if (a.modified_at > local[a.slug]
                      || (a.modified_at == 0 && local[a.slug] == 0)) updates.push(a.slug);
              } catch(err) {
                  console.error(err);
              }
          }
          return Promise.resolve(updates);
      }
docs: Prepares a list of items that are eligible for updates.
  Note: This is different than what's available for download. Everything in the index is technically available for download.
  
  Eligibility is determined by:
  * available items who's modified_at exceeds the local equivalent
  * available items who's modified_at is 0 and accompanying local item's modified_at is 0
  
  @param local {{}} keyed object of local (downloaded) items.
  @param available {[{slug, modified_at}]} an array of available (indexed) items
  @returns {Promise.<Array>}