Labs 5 - Assignment discussion.
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1. For each of the following cases discuss the expected results, possible 
   erroneous results and how to ensure correctness:

   - read committed, single-step booking
   - read committed, two-steps booking
   - serializable, single-step booking
   - serializable, two-steps booking

2. Address what went good and what went wrong in your implementation (be 
   self-critical).

3. Address what one has to be aware of while implementing transactions.