public Tensor.Builder block(TensorAddress sparsePart, double[] values) {
            int denseSubspaceSize = (int)denseSubspaceSize();
            if (values.length < denseSubspaceSize)
                throw new IllegalArgumentException("Block should have " + denseSubspaceSize +
                                                   " values, but has only " + values.length);
            double[] denseSubspace = denseSubspace(sparsePart);
            System.arraycopy(values, 0, denseSubspace, 0, denseSubspaceSize);
            return this;
        }
For each sparse partial address, hold a dense subspace
vespajlib/src/main/java/com/yahoo/tensor/MixedTensor.java