code: function removeMirageIndices(countMax, indexFirstKnown) {
                      var placeholdersAtEnd = 0;
  
                      function removePlaceholdersAfterSlot(slotRemoveAfter) {
                          for (var slot2 = slotListEnd.prev; !(slot2.index < countMax) && slot2 !== slotRemoveAfter;) {
                              var slotPrev2 = slot2.prev;
                              if (slot2.index !== undefined) {
                                  deleteSlot(slot2, true);
                              }
                              slot2 = slotPrev2;
                          }
  
                          placeholdersAtEnd = 0;
                      }
  
                      for (var slot = slotListEnd.prev; !(slot.index < countMax) || placeholdersAtEnd > 0;) {
                          var slotPrev = slot.prev;
  
                          if (slot === slotsStart) {
                              removePlaceholdersAfterSlot(slotsStart);
                              break;
                          } else if (slot.key) {
                              if (slot.index >= countMax) {
                                  beginRefresh();
                                  return false;
                              } else if (slot.index >= indexFirstKnown) {
                                  removePlaceholdersAfterSlot(slot);
                              } else {
                                  if (itemsFromKey) {
                                      fetchItemsFromKey(slot, 0, placeholdersAtEnd);
                                  } else {
                                      fetchItemsFromIndex(slot, 0, placeholdersAtEnd);
                                  }
  
                                  // Wait until the fetch has completed before doing anything
                                  return false;
                              }
                          } else if (slot.indexRequested || slot.firstInSequence) {
                              removePlaceholdersAfterSlot(slotPrev);
                          } else {
                              placeholdersAtEnd++;
                          }
  
                          slot = slotPrev;
                      }
  
                      return true;
                  }
docs: Removes any placeholders with indices that exceed the given upper bound on the count