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Changes between Initial Version and Version 1 of Ticket #371

initial	v1
1	1	The target graph must be cleared before graph copying. The `clear()` function must be used in the `DigraphCopySelector` and `GraphCopySelector` classes before adding nodes and arcs/edges.
2	2
3		It is a question if `build()` is required to c~~all~~ lear the graph or we should explicitly call `clear()` before `build()` for those structures that provides `build()` function.
	3	It is a question if `build()` is required to clear the graph or we should explicitly call `clear()` before `build()` for those structures that provides `build()` function.