private DenseSubspace blockOf(TensorAddress address) {
            TensorAddress sparsePart = address.mappedPartialAddress(sparseType, type.dimensions());
            Integer blockNum = sparseMap.get(sparsePart);
            if (blockNum == null || blockNum >= denseSubspaces.size()) {
                return null;
            }
            return denseSubspaces.get(blockNum);
        }
An immutable index into a list of cells.
Contains additional information required
for handling mixed tensor addresses.
Assumes indexed dimensions are bound.
vespajlib/src/main/java/com/yahoo/tensor/MixedTensor.java