return {
  _new = (function(val) return function() return {value = val} end end),
  newWithSelf = (function(f)
    return function()
      local ref = {value = nil}
      ref.value = f(ref)
      return ref
    end
  end),
  read = (function(ref) return function() return ref.value end end),
  modifyImpl = (function(f)
    return function(ref)
      return function()
        local t = f(ref.value)
        ref.value = t.state
        return t.value
      end
    end
  end),
  write = (function(val) return function(ref) return function() ref.value = val end end end)
}