# File lib/Borges/Utilities/WeakIdentityKeyHash.rb, line 135
    def self.by_obj(obj)
      ref = nil

      Thread.exclusive do
        rids = ID_MAP[obj.object_id]

        if rids.nil? or rids.empty? then
          raise RefError, "Reference does not exist"
        end

        for rid in rids do
          begin
            ref = ObjectSpace._id2ref rid
          rescue RangeError
            next
          end

          return ref unless ref.nil?
        end

        raise RefError, "Reference does not exist"
      end
    end

  end