Answer: Colorado State University extension provides an excellent summary of your options on the tree stump problem at the following web site.
To grow lawn grass over the cement walk, you will unfortunately have to remove the walk before you plant grass in that area. If you don't, the lawn will always look "different" and show the pattern of the walk; it will dry out faster than the rest of your yard and the grass will be less healthy there, too. (You would need about eight inches or soil or more on top of it to allow for healthy grass roots.)
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