interface IBStack<A> extends IStackWithOptions {
  // record an undoable action for backtracking
  void addUndo aka temp(AutoCloseable undo);
  
  // initiate a backtrack
  IBStack<A> backtrack();
  
  // get next result of computation
  A nextResult();
  A nextResultWithPrintStruct(long maxSteps);
}

Began life as a copy of #1035394