Clarification re: comments about relative importance of ideas..

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Thank you for saying "This is the first non-trivial thing you have learned..." Explicitly highlighting the paramount ideas is extremely helpful when learning a new subject.
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At the risk of looking at the gift horse in the mouth let me add a clarification to this person as well as the rest of the class.
Note that what I said about "IDDFS" is that it is the first non-trivial *specific* idea you have learned (check out the recording ;-))
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The whole discussion on environment and problem types is actually much deeper, but more general (so it is non-trivial *general* thing).

As CS students you are already well-trained to appreciate specific algorithmic ideas ( e.g. IDDFS)--and you will have more than enough opportunities
to exercise that specific form of appreciation as we move forward.

However, understanding the problem types and connection to environment properties is something that you might want to learn to appreciate.
You should definitely read chapters 2 and 3 carefully.