code: func (dbi *dbItem) Less(item btree.Item, ctx interface{}) bool {
  	dbi2 := item.(*dbItem)
  	switch ctx := ctx.(type) {
  	case *exctx:
  		// The expires b-tree formula
  		if dbi2.expiresAt().After(dbi.expiresAt()) {
  			return true
  		}
  		if dbi.expiresAt().After(dbi2.expiresAt()) {
  			return false
  		}
  	case *index:
  		if ctx.less != nil {
  			// Using an index
  			if ctx.less(dbi.val, dbi2.val) {
  				return true
  			}
  			if ctx.less(dbi2.val, dbi.val) {
  				return false
  			}
  		}
  	}
  	// Always fall back to the key comparison. This creates absolute uniqueness.
  	if dbi.keyless {
  		return false
  	} else if dbi2.keyless {
  		return true
  	}
  	return dbi.key < dbi2.key
  }
docs: // Less determines if a b-tree item is less than another. This is required
  // for ordering, inserting, and deleting items from a b-tree. It's important
  // to note that the ctx parameter is used to help with determine which
  // formula to use on an item. Each b-tree should use a different ctx when
  // sharing the same item.