question-1: 
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You should assume that the '_data' Object of the node in the Graph is Copiable. 

question-3:
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- Clarification: Gui.start() activates a mechanism (which is called 'timer') inside Gui, that calls App.advance() every certain time interval. The mechanism continues until Gui.stop() is called. The idea is that the function advance() will cause a *single* move of *a* ball. 

- The velocities of the balls should be 1,2,..,n.