Tuesday (Arc-Day)

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// TODO add Construct_(planar_)arc_2 to CKvA_2, make
   constructors protected and add pointers to constructing
   CKva_2-instance in Arc_2

* Introduce Construct_arc_2l


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// TODO put all functionality into functors and make the other private
   members static and functors friend

* Functors with caches
- Compare_x_near_boundary
- Compare_y_at_x

* Templatize functors and virtualize members

* Other Functors
- Is_in_x_range

- Correctness of Is_bounded 

- Correctness of same_arc_compare_xy

* remove Curve_kernel_2 in Arc_2_base


* Order of (static) members

* remove Arc_base?

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// TODO Adaptions for QdX

- Rewrite Intersect and other predicates for QdX

- TODO: remove mult in Intersect for QdX as it might does not make sense?


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// TODO Filtered CKvA_2

* Derive CKvA, replace Intersect_2 to obtain Filtered_CKvA_2