code: public static <T> Func1<Iterable<T>, SparseArray<T>> toSparseArray(Func1<T, Integer> itemToKey) {
          return toSparseArray(itemToKey, 10);
      }
docs: Returns a method that can be used with {@link solid.stream.Stream#collect(Func1)}
  to convert a stream into a {@link SparseArray}.
  
  @param <T>       a type of stream items.
  @param itemToKey a method that should return a key for an item.
  @return a method that converts an iterable into a {@link SparseArray}.