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MessageQueue: Message Queues

class MessageQueue 
{
  MessageQueue()
  boolean isEmpty()
  void enqueue(Message m)
  Message dequeue()
  MessageCell getCell()
}

This type is used to implement message selectors.

A message queue is a FIFO (first in/first out) list of messages. The default constructor creates an empty queue; isEmpty returns true if and only if a queue is empty; enqueue adds a message to the end of the queue; dequeue extracts the head of the queue (removing it from the queue).

For low-level queue manipulation, getCell returns the cell representing the head of the queue (or null, if the queue is empty).