by_obj (Weak::Key)

    # 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