@Override
        public MixedTensor build() {
            // TODO: This can be solved more efficiently with a single map.
            Set<Map.Entry<TensorAddress, double[]>> entrySet = denseSubspaceMap.entrySet();
            for (Map.Entry<TensorAddress, double[]> entry : entrySet) {
                TensorAddress sparsePart = entry.getKey();
                double[] denseSubspace = entry.getValue();
                var block = new DenseSubspace(sparsePart, denseSubspace);
                indexBuilder.addBlock(block);
            }
            return new MixedTensor(type, indexBuilder.build());
        }
For each sparse partial address, hold a dense subspace
vespajlib/src/main/java/com/yahoo/tensor/MixedTensor.java