@Nullable
  private static Ref<Change> doBuildChangesFast(int length1, int length2, int startShift, int endCut) {
    int trimmedLength1 = length1 - startShift - endCut;
    int trimmedLength2 = length2 - startShift - endCut;
    if (trimmedLength1 != 0 && trimmedLength2 != 0) return null;
    Change change = trimmedLength1 != 0 || trimmedLength2 != 0 ?
                    new Change(startShift, startShift, trimmedLength1, trimmedLength2, null) :
                    null;
    return new Ref<>(change);
  }
@Nullable
  private static Ref<Change> doBuildChangesFast(int length1, int length2, int startShift, int endCut)