private static HashSet<TensorAddress> findSubspacesNotInInput(Tensor input, Tensor modifier) {
        var subspaces = new HashSet<TensorAddress>();
        var inputCells = input.cells();
        var type = input.type();
        for (var itr = modifier.cellIterator(); itr.hasNext(); ) {
            Tensor.Cell cell = itr.next();
            TensorAddress address = cell.getKey();
            if (!inputCells.containsKey(address)) {
                subspaces.add(createSparsePartAddress(address, type));
            }
        }
        return subspaces;
    }
Converts the given tensor type to a type that is compatible for being used in this update (has only mapped dimensions).
document/src/main/java/com/yahoo/document/update/TensorModifyUpdate.java