What are those things that look like concatenated hourglasses in the included (TTL) circuit?

What are those things that look like concatenated hourglasses in the (TTL) circuit of Question 3? https://www.docdroid.net/1L1FeI3/question3.pdf I'm trying to fully understand how to solve that problem, but I don't even know what to search for, hence this question. Any input would be GREATLY appreciated! show more What are those things that look like concatenated hourglasses in the (TTL) circuit of Question 3?

https://www.docdroid.net/1L1FeI3/questio...

I'm trying to fully understand how to solve that problem, but I don't even know what to search for, hence this question.

Any input would be GREATLY appreciated!
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