Dask-geomodeling; Quick start is not "quick"

dask_geomodeling_quick_not_quick.py

import dask
import dask_geomodeling
request = {
    "mode": "vals",
    "bbox": (138000, 480000, 139000, 481000),
    "projection": "epsg:28992",
    "width": 256,
    "height": 256
}
graph, name = add.get_compute_graph(**request) 
data = dask.get(graph, [name])

"""

Output
-----------
Error: add is not defined

Description
-----------
Looking through the documentation, I came across this;
https://github.com/nens/dask-geomodeling/blob/master/docs/blocks.rst
Based on this, add could be.

"""
  from dask_geomodeling.raster import RasterFileSource
  add = RasterFileSource('path/to/geotiff')

"""
This could of course be wrong because the documentation is not clear. 
If I define add using RasterFileSource, this throws a different error
where the tif cannot be found but this is explicitly defined.
Please can you produce a reproducible example.
"""