Where you start does not determine where you finish
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Example showing propagation of child nodes.
Initially discs 1 to 3, 4 to 5, and 6 are grouped.
The first group passes disc 1 (but could pass disc 2 or no disc) to the second group ( which has one spare child node).
The second group passes disc 1 (but could pass disc 4 or no disc) to the third group (which has its one available child node spare)
The third group has no option to pass on any child disc
The reverse process has no option but to return any child nodes to their original parent node.