nsIJSContextStackIterator Interface

 Returns true if this iterator is at the end of its stack's contexts.
 @throws NS_ERROR_NOT_INITIALIZED If there has not been a previous call
         to reset.

boolean done()

 Returns the prev JSContext off of stack. Note that because we're 
 iterating over a stack, this value would be the next popped value.

 @throws NS_ERROR_NOT_INITIALIZED If there has not been a previous call
         to reset.
 @throws NS_ERROR_NOT_AVAILABLE if already at the end.

JSContext prev()

 Resets this iterator to the beginning of this thread's stack.

void reset(in nsIJSContextStack stack)