Object removedValue;

    if (previousValue == null || previousValue == PRED) {
      removedValue = null;
    } else if (previousValue instanceof PredAndSucc) {
      adjacentNodeValues.put((N) node, PRED);
      removedValue = ((PredAndSucc) previousValue).successorValue;
    } else { // successor
      adjacentNodeValues.remove(node);
      removedValue = previousValue;
    }