if (VM.VerifyAssertions) VM_Scheduler.assert(t.next == null);
if (VM.VerifyAssertions) VM_Scheduler._assert(t.next == null);
  public void enqueue (VM_Thread t) {    if (VM.BuildForEventLogging && VM.EventLoggingEnabled) VM_EventLogger.logEnqueue(t, id);    if (VM.VerifyAssertions) VM_Scheduler.assert(t.next == null); // not currently on any other queue    if (head == null)      head = t;    else      tail.next = t;    tail = t;  }