AdjacencyIterator::isBegin() and AdjacencyIterator::isEnd() always
returned a False value and printed a "not implemented" warning message.
This caused an infinite loop in a few chaining iterators, resulting in
a crash of the program. The origin of the issue seemed to be a fact
that in the C++ layer, the AdjacencyIterator class had not properly
overloaded the definitions of the methods in the Iterator superclass.
The fix here looks okay, although I'm not sure why this inconsistency
was not addressed for a long time.
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