CacheEntry = object
  mid*: MessageID
  topicIDs*: seq[string]

MCache = object of RootObj
  msgs*: Table[MessageID, Message]
  history*: seq[seq[CacheEntry]]
  windowSize*: Natural

func contains(c: MCache; mid: MessageID): bool {....raises: [Defect], tags: [].}

func get(c: MCache; mid: MessageID): Option[Message] {....raises: [Defect],
    tags: [].}

func init(T: type MCache; window, history: Natural): T:type {....raises: [Defect].}

func put(c: var MCache; msgId: MessageID; msg: Message) {....raises: [Defect],
    tags: [].}

func shift(c: var MCache) {....raises: [Defect], tags: [].}

func window(c: MCache; topic: string): HashSet[MessageID] {....raises: [Defect],
    tags: [].}