code: public final Ix<Ix<T>> window(int size, int skip) {
          if (size == skip) {
              return window(size);
          }
          if (size < skip) {
              return new IxWindowSkip<T>(this, positive(size, "size"), positive(skip, "skip"));
          }
          return new IxWindowOverlap<T>(this, positive(size, "size"), positive(skip, "skip"));
      }
docs: Emits inner Ix Iterables of potentially overlapping sequences mapped from this
  sequence with the given maximum size each and started each {@code skip} source elements.
  @param size the maximum size of the inner windows, positive
  @param skip after how many elements to start a new window (repeatedly), positive
  @return the new Ix instance
  @throws IllegalArgumentException if size or skip is non-positive
  @since 1.0
  @see #window(int)