@Override
        public SubspaceIterator next() {
            if ( ! hasNext()) throw new NoSuchElementException("No cell at " + superindexes);
            count++;
            superindexes.next();
            return new SubspaceIterator(subdimensionIndexes, superindexes.indexesCopy(), iterateSizes);
        }
The sizes of the space we'll return values of, one value for each dimension of this tensor,
which may be equal to or smaller than the sizes of this tensor
vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java