def each
      @references.each do |key, ref|
        value = ref.object
        yield(key, value) if value
      end
    end
def each
      @references.each do |key, ref|
        value = ref.object
        yield(key, value) if value
      end
    end
Iterate through all the key/value pairs in the map that have not been reclaimed
 by the garbage collector.