def delete(key)
      ref = @references.delete(key)
      if ref
        keys_to_id = @references_to_keys_map[ref.referenced_object_id]
        if keys_to_id
          keys_to_id.delete(key)
          @references_to_keys_map.delete(ref.referenced_object_id) if keys_to_id.empty?
        end
        ref.object
      else
        nil
      end
    end
def delete(key)
      ref = @references.delete(key)
      if ref
        keys_to_id = @references_to_keys_map[ref.referenced_object_id]
        if keys_to_id
          keys_to_id.delete(key)
          @references_to_keys_map.delete(ref.referenced_object_id) if keys_to_id.empty?
        end
        ref.object
      else
        nil
      end
    end
Remove the entry associated with the key from the map.