def delete(key)
      @lock.synchronize do
        rkey = ref_key(key)
        if rkey
          @references_to_keys_map.delete(rkey)
          @values.delete(rkey)
        else
          nil
        end
      end
    end
def delete(key)
      @lock.synchronize do
        rkey = ref_key(key)
        if rkey
          @references_to_keys_map.delete(rkey)
          @values.delete(rkey)
        else
          nil
        end
      end
    end
Remove the value associated with the key from the map.