code: func (h SlotHandle) Fetch(c context.Context, cb FetchCallback) (interface{}, error) {
  	return c.Value(&processCacheKey).(*ProcessCacheData).slots[int(h)].Get(c, lazyslot.Fetcher(cb))
  }
docs: // Fetch returns the cached data, if it is available and fresh, or attempts to
  // refresh it by calling the given callback.
  //
  // If the context doesn't have ProcessCacheData installed, this will panic.