When a Graph contains cycles, e.g. 1->2->3->1 _layering would end
up in an infinite loop since IS_SUBSET would never be true.
By keeping check of the size of z, which contains the already layered
nodes, one can detect a freeze (since it should change after
current_layer increases. If it doesn't "u" didn't change and q and u
will never be equal resulting in a freeze/infinite while loop).
If a freeze happens warn the user and put all the nodes part of the
cycle (leftover in p) to the first layer which will end up in them
being listed top to bottom.
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