"Now, what does it mean to delete a row? "
To delete a row that was later thought to be a redundant for the application.
"Suppose it is the second row that is to be deleted. What function value takes the place of "b" on the interval [1,2)? I think that's the essential question."
No that is not the essential question, the question is simply that; if you want to delete a row or in other words edit your piecewise function. What would one do. Go back, bring the cursor to the specific row and select the row and delete it. That is the most intuitive process and should be the most basic step to perform to achieve the desired result.
" it is recognized that this row is not wanted. Control-z will undo the insertion of the row."
Not a thorough solution process for all situations. Execute the input and go back and delete a row and try re-executing it, it would throw an error. Why?? Because Maple doesnt understand that the user decided to undo a row.