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This allows parts of the graph that do not belong to the fragment to not
be iterated over when optimizing said fragment.

The original code would only record vertices that were reached in two
different ways (and thus the edge was already marked as black when
visited a second time).

Nodes should be rejected based on the existence of an edge, not based on
whether that edge has already been visited or not.

value to a clique

iterating over them

An updatable priority queue is required to change the keys of elements
if the priority queue when making decisions or backtracking, hence why
std::priority_queue was not appropriate.

A monotonic clock (e.g. clock_gettime(2), GetTickCount64) would be more
appropriate.

Changing (most of) these parameters from their default values would not
have worked without adding explicit template parameters to
e.g. recursive calls.