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Project $1$: Quad$-$trees

; total$-$rect$-$area: returns the total integer area covered by any $($one or more$)$ rectangles in the given list

; Don't double$-$count. Your solution must be in O$($n log n$).$

Hint: implement an immutable quad$-$tree to represent $2$D space; FYI, a solution takes only ~$90$ lines of code. $($You could also use a sorting and scanning approach.$)$

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